Biosensors for monitoring receptor-mediated G-protein activation

ABSTRACT

The present invention relates to novel biosensors that are based on bioluminescence resonance energy transfer (BRET). These biosensors may be used to monitor rapid interaction and conformational changes within G protein-coupled receptor/G protein complexes and, in this way, reflect the activation status of the receptor. Advantageously, the biosensors may be used as a highly sensitive and quantitative assay for the identification of ligands (agonists, antagonists, inverse agonists, partial agonists, etc.) targeting G protein-coupled receptors (GPCRs) as well as for the analysis of the activation status of these receptors. Moreover, multiplexing different biosensors within receptors/G protein complexes allows for mapping ligand textures. Additionally, the biosensors permit the direct, real-time examination of interactions between receptors and G protein in their natural environment, the living cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No. PCT/CA2006/00233, filed Feb. 16, 2006, which claims priority from U.S. Provisional Application No. 60/653,126, filed on Feb. 16, 2005, both of which are hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to novel biosensors. Specifically, the invention relates to biosensors that are based on bioluminescence resonance energy transfer (BRET). These biosensors may be used to monitor rapid interaction and conformational changes within G protein-coupled receptor/G protein complexes and, in this way, reflect the activation status of the receptor. Advantageously, the biosensors may be used as a highly sensitive and quantitative assay for the identification of ligands (agonists, antagonists, inverse agonists, partial agonists, etc.) targeting G protein-coupled receptors (GPCRs) as well as for the analysis of the activation status of these receptors. Moreover, multiplexing different biosensors within receptors/G protein complexes allows for mapping ligand textures. Additionally, the biosensors permit the direct, real-time examination of interactions between receptors and G protein in their natural environment, the living cell.

BACKGROUND OF THE INVENTION

G-protein-coupled-receptors (GPCRs) also known as 7 transmembrane receptors (7TM) represent the largest family of cell surface receptors involved in signal transduction across biological membranes. They control a large diversity of physiological processes including vision, taste, olfaction, neuronal and hormonal transmission, cell growth and metabolism. Consequently, these receptors are important targets for the development of drugs with wide clinical applications. In recent years, increased knowledge about GPCR has facilitated the development and screening of many new therapeutically active molecules. However, our knowledge concerning the molecular events determining receptor signaling efficacy remains rudimentary.

Although recent advances in cellular biology have led to the identification of a broad range of proteins directly interacting with GPCRs¹, coupling with αβγ trimeric G proteins remains the common benchmark of all GPCR family members. Thus, these proteins constitute one of the most important and earlier plasma membrane transducers relaying information from activated cell surface GPCRs to intracellular signaling molecules. It is usually admitted that agonist binding promotes or stabilizes specific conformational states of the receptors that favor the engagement of the Gαβγ complex by specific receptor domains. The ensuing exchange of guanosine diphosphate (GDP) for guanosine triphosphate (GTP) on the Gα subunit is then believed to precipitate complex disassembly, leading to free GTP-bound Gα and Gβγ subunits that can in turn interact and modulate downstream effectors. The termination of the signal results from the re-association of the holo-Gαβγ inactive heterotrimer following GTP hydrolysis by the Gα subunit¹⁻³. However, this generally-accepted classical collision-based model, deduced largely from in vitro studies, has recently been challenged. Indeed, in vitro reconstitution assays⁴, genetic studies in yeast⁵ and resonance energy transfer studies carried out in living cells⁶⁻⁸ have suggested that stable receptor/G protein and/or heterotrimeric G protein complexes may persist during the activation process. Information about these putative complexes and the structural rearrangement underlying their activation remain however rudimentary.

Resolution of crystal structures for active (GTP- or GTPγS-bound), inactive (GDP-bound) and transition states (GDP-AIF⁴⁻ bound) of several Gα subunits and of the inactive form of the Gα_(i1)β₁γ₂ and Gα_(t)βγ_(t) heterotrimeric complexes has provided an initial basis for understanding the structural rearrangements involved in G protein activation⁹. However, the structural differences between active and inactive conformers are relatively modest, involving only local differences in the switch I, switch II and switch III regions that play key roles in guanine nucleotide exchange^(10, 11). Although models of activation have been proposed based on these three dimensional crystal structures, on analogy with the small G proteins^(12, 13) and on site-directed mutagenesis studies or natural mutations found in diseases^(2, 3, 14, 15), the static nature of the crystals and the absence of information about the empty state of Gα limit the understanding of the dynamic changes occurring during receptor-promoted G protein activation.

At present, no method permits the direct assessment of the real-time interactions between receptors and G protein in living cells. Such assays would be particularly relevant given that the dynamics of protein interactions can be influenced by multiple spatio-temporal factors that cannot be easily recreated using in vitro assays. They would also provide new tools to test the ability of compounds to modulate the early steps in the signaling pathway, thus facilitating the identification of potential drug candidates.

There is therefore a need for biosensors that will allow the determination or assessment of the early steps in GPCR signaling and that may also serve to identify new ligands (agonists, antagonists, reverse agonists, partial agonists, etc.) for these receptors with defined signaling efficacy.

The present invention seeks to meet this and related needs.

SUMMARY OF THE INVENTION

In recent years, fluorescence and bioluminescence resonance energy transfer approaches (FRET and BRET) have been increasingly used to study protein-protein interactions and appreciate dynamic changes in protein conformation^(16, 17). The dependence of the energy transfer efficacy on the distance between energy donors and acceptors permits real time measurements that are both sensitive and specific to the labelling sites of the proteins thus allowing inference on the dynamic structural changes¹⁸⁻²².

Taking advantage of multiple sites of energy donor and acceptor insertions in the protein-protein complex of interest, the present invention relies on the development of a BRET-based assay that directly monitors real-time interactions between GPCRs and their cognate G proteins and among the G protein subunits in living cells. In addition to monitor pre-assembled receptor/G protein complexes (reflected by basal BRET signal), the assay is capable of monitoring (as assessed by changes in the BRET signal detected in the assay) ligand-modulated interactions between receptor and G protein subunits (i.e., Gα, Gβ and Gγ subunits) reflecting ligand-modulated G protein coupled receptor activation and G protein engagement as well as ligand-modulated interactions between G protein subunits reflecting G protein activation.

In addition, the invention provides a useful tool to probe for conformational changes occurring in the receptor/G protein complexes and G protein heterotrimer resulting from ligand binding to the receptor. As a result, by multiplexing different BRET-biosensors of the receptor/G protein or the G protein complex itself, the invention offers the possibility to set up pharmacological fingerprints that are specific to each receptor ligand, thus allowing to differentiate the distinct signalling modes of different ligand toward the various signalling pathways engaged.

Results demonstrate that GPCR fused to the energy donor Renilla luciferase (RLuc) and G protein subunits attached to a green fluorescent protein (YFP, GFP10 or GFP²) energy acceptor or vice versa, provide reliable biosensors that can directly monitor the kinetic and selectivity of G protein engagement upon receptor activation. In addition to offering a new generally applicable method to probe dynamic protein interactions involved in G protein activation, the approach offers an advantageous tool to monitor the activation of distinct G protein subunits using a single assay mode. Receptor-mediated G protein activation can also be detected through a biosensor using the Gβ or Gγ subunits fused to RLuc and the Gα subunit fused to GFP10, GFP2 or YFP and/or vice versa, thus allowing to monitor interactions between Gα and Gβ or Gγ subunits fusion proteins as a result of activation of an untagged receptor. The data described herein provides direct kinetic measurement of the receptor-mediated activation of heterotrimeric G proteins in living cells, allowing the detection of both activation and desensitization events.

Due to its sensitivity and applicability for all major classes of Gα subunits (Gαs, Gαi/o, Gαq/11, Gα12/13), the receptor/Gβγ interaction BRET assay can be viewed as a general method to monitor receptor-mediated G protein activation. In that respect, it may be considered superior to the widely used GTPγS binding assay since the nucleotide binding method has proven difficult for Gs and Gq coupled receptor due to the very weak signal/noise ratio obtained with these Gα subunits. When compared with the classical second messenger-based read-out that are classically used to identify the G proteins that can be activated by a given receptor, the BRET-based assays present the advantage of a single and homogeneous assay mode that directly measure the engagement of the G protein. Thus the receptor/G protein interaction BRET assay could advantageously be used as a general strategy for the identification of ligands for GPCRs.

Directly monitoring the interaction between various receptors (fused to GFP2, GFP10 or YFP) and distinct Gα subunits (fused to Rluc) or between distinct Gα subunits (fused to Rluc) and various Gβγ subunits (fused to GFP2, GFP10 or YFP) also offers a tool to directly monitor the selectivity of interactions between the signaling partners in the response to a specific ligand (drug).

Even though the present invention has been exemplified through different receptor and G protein subunit constructs, seven of them have been newly generated and found to be useful for ligand texture fingerprinting in addition to the purposes described above:

phRluc-Gγ₂—The HindIII-XbaI fragment of pcDNA3.1-humanGγ₂ (Güthrie Research Institute, PA) was excised by double digestion with HindIII-XbaI and then subcloned into the HindIII-XbaI-digested humanized pRluc-C1 vector (Perkin Elmer, Lifescience). The final phRluc-Gγ₂ expression vector will encode the human Gγ₂ of heterotrimeric G protein subunit fused to its N-terminus to the humanized luciferase.

pGFP²-Gγ₂—The HindIII-XbaI fragment of pcDNA3.1-humanGγ₂ (Güthrie Research Institute, PA) was excised by double digestion with HindIII-XbaI and then subcloned into the HindIII-XbaI-digested humanized pGFP²—C3 vector (Perkin Elmer, Lifescience). The final pGFP²-Gγ₂ expression vector will encode the human Gγ₂ of heterotrimeric G protein subunit fused to its N-terminus to the Green Fluorescent protein variant GFP².

phRluc-Gβ₁—The HindIII-XbaI fragment of pcD NA3.1-humanGβ₁ (Güthrie Research Institute, PA) was excised by double digestion with HindIII-XbaI and then subcloned into the HindIII-XbaI-digested humanized pRluc-C1 vector (Perkin Elmer, Lifescience). The final phRluc-Gβ₁ expression vector will encode the human Gβ₁ of heterotrimeric G protein subunit fused to its N-terminus to the humanized luciferase.

pGFP²-Gβ₁—The HindIII-XbaI fragment of pcDNA3.1-humanGβ₁ (Güthrie Research Institute, PA) was excised by double digestion with HindIII-XbaI and then subcloned into the HindIII-XbaI-digested humanized pGFP²-C3 vector (Perkin Elmer, Lifescience). The final pGFP²-Gβ₁ will encode the human Gβ₁ of heterotrimeric G protein subunit fused to its N-terminus to the Green Fluorescent protein variant GFP².

pcDNA3.1-Gα_(i1)-60Rluc-pcDNA3.1-Gα_(i1)-91Rluc-pcDNA3.1-Gα_(i1)-122Rluc—Coding sequence of humanized Rluc (PerkinElmer, Lifescience) was PCR amplified without its STOP and inserted via flexible linkers (SGGGGS) in the coding sequence of human Gα_(i1) (pcDNA3.1-human Gα_(i1), Güthrie Research Institute, PA) between: residues L91 and K92 (Gα_(i1)-91Rluc, corresponding to Sequence ID No. 2) or residues E122 and L123 (Gα_(i1)-122Rluc, corresponding to Sequence ID No. 3) or residues G60 and Y61 (Gα_(i1)-60Rluc, corresponding to Sequence ID No. 1). The final pcDNA3.1-Gα_(i1)-91Rluc and pcDNA3.1-Gα_(i1)-122Rluc expression vectors will encode the human Gα_(i1) of heterotrimeric G protein subunit fused to its helical domain to the humanized luciferase, while the pcDNA3.1-Gα_(i1)-60Rluc expression vector will encode the human Gα_(i1) of heterotrimeric G protein subunit fused to its linker 1 region to the humanized luciferase.

Thus, in addition to shedding new light on the dynamics of receptor-mediated G protein activation, the BRET-based biosensors of the present invention can be used to directly probe the selectivity of interaction between receptors and G protein subunits and therefore offers a sensitive assay to monitor receptor-mediated G protein activation. The invention therefore provides a sensitive assay for the screening of candidate drugs acting on specific G protein coupled receptors, which may be incorporated in a kit for sale or distribution.

Other objects, advantages and features of the present invention will become apparent upon reading of the following n on restrictive description of preferred embodiments thereof, given by way of example only with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: Illustration of the receptor/G protein BRET2 assay. BRET was measured between receptor-Rluc and Gαs-GFP10 (FIG. 1-1), GFP10-Gγ2 (FIG. 1-3) or GFP10-Gβ1 FIG. 1-2 as indicated. Upon degradation of its substrate (DeepBlueC coelenterazine), the Rluc fused to the C-terminus of the receptor emits a blue light with an emission peak at 395 nm. When receptor and G protein are in close proximity (<100 Angstroms), a non-radiative transfer of energy will occur between Rluc and GFP10 fused to the different G protein subunits, resulting in re-emission of fluorescence with a peak at 510 nm.

FIG. 2: BRET measurements of GPCRs/Gαsβ1γ2 interactions in living cells. GFP10 is referred to as GFP in the figure. FIG. 2A: BRET measured in cells co-expressing Rluc-tagged β2 adrenergic receptor (β2AR-Rluc) with either Gαs-GFP10 (FIG. 2A-1), GFP10-Gβ1 (FIG. 2A-2), or GFP10-Gγ2 (FIG. 2A-3), in the presence of the indicated G protein subunits, and stimulated (▪) or not (□) with 10 μM Iso. FIG. 2B: BRET measured in cells co-expressing Rluc tagged-Gα_(i1) (Gα_(i1)-Rluc) and GFP tagged-β₂ adrenergic receptor (β₂AR-GFP10) only, in the presence (▪) or not (□) of 10 μM Iso. Data represent the mean±SEM of 3-4 independent experiments. FIG. 2C: BRET measured between GFP-Gβ1 (FIGS. 2C-1, 2C-3, 2C-5, and 2C-7) or GFP-Gγ2 (FIGS. 2C-2, 2C-4, 2C-6, and 2C-8) and Rluc-tagged V2 vasopressin receptor (V2R-Rluc), α_(2A) adrenergic receptor (α_(2A)AR-Rluc), sensory neuron-specific receptor 4 (SNSR4-Rluc) and thromboxane A2α receptor (TPα-Rluc), as indicated, after coexpression of the indicated G protein subunits, and in the presence of their respective selective agonists (AVP, UK14304, BAM22 and U46619). Results are expressed as the difference in the BRET signal observed in the presence and the absence of agonists. FIG. 2D: BRET measured in cells co-expressing β2AR-Rluc or CD8-Rluc and GFP10-Gβ1 (+Gγ2+Gαs, FIG. 2D-1) or GFP10-Gγ2 (+Gβ1+Gαs, FIG. 2D-2) and stimulated (▪) or not (□) with 10 μM Iso. Data represent the mean±SEM of 3-4 independent experiments. *, p<0.05. #, p<0.05 compared with Iso induced BRET in the absence of co-transfected Gα subunit (dashed line).

FIG. 3: BRET signals between β2AR and Gβγ reflect receptor-mediated G protein activation. GFP10 is referred to as GFP in the figure. FIG. 3A: BRET measured in HEK293T cells co-expressing β2AR-Rluc with either GFP10-Gβ1 (FIG. 3A-1) or GFP10-Gγ2 FIG. 3A-2 in the presence of increasing concentrations of Iso. FIG. 3B: cAMP production measured in HEK293T cells expressing β2AR in the presence of increasing concentrations of Iso. FIG. 3C: BRET measured in HEK293T cells coexpressing β2AR-Rluc and GFP10-Gβ1 (FIG. 3C-1) or GFP10-Gγ2 (FIG. 3C-2) in the absence (basal) or presence of the indicated β-adrenergic ligands. FIG. 3D: cAMP production measured in HEK293T cells expressing β2AR in response to stimulation with the indicated ligands. FIG. 3E: BRET measured in HEK293T cells coexpressing Gα_(i1)-Rluc and α_(2A)AR-GFP10, in the absence (basal) or presence of the indicated α-adrenergic ligands (10 μM UK14304, RX821002, Yohimbine, Dexmetodine, Clonidine). FIG. 3F: cAMP production measured in HEK293T cells expressing β2AR-Rluc or D79N-β2AR-Rluc in the absence (□) or presence (▪) of Iso. FIG. 3G: [¹²⁵I]-CYP competition binding by Iso in membranes derived from cells expressing β2AR-Rluc (Δ) or D79N-β2AR-Rluc (▴). Data are expressed as the percentage of maximal specific binding. FIG. 3H: BRET measured in HEK293T cells expressing β2AR-Rluc or D79N-β2AR-Rluc with either GFP10-Gβ1 (FIG. 3H-1) or GFP10-Gγ2 (FIG. 3H-2), in the absence (□) or presence (▪) of Iso. FIG. 3I: BRET measured in the absence (□) or presence (▴) of Iso in membrane preparations derived from HEK293T cells co-expressing β2AR-Rluc with either GFP10-Gβ1 (FIG. 3I-1) or GFP10-Gγ2 (FIG. 3I-2), and pretreated (GTPγS) or not (NT) with GTPγS during 90 min at 25° C. Data represent the mean±SEM of 3-4 independent experiments each performed in duplicate. *, p<0.05. Treatments: Iso, 10 μM; Feno, 10 μM; ICI, 100 μM; GTPγS, 200 μM.

FIG. 4: Kinetics analysis of β2AR/Gβ1γ2 interactions. GFP10 is referred to as GFP in the figure. FIG. 4A: BRET measured every 0.05 seconds for 10 seconds in cells expressing β2AR-Rluc and GFP10-Gβ1 (filled triangle) or GFP10-Gγ2 (open triangle). Iso (10 μM) was injected 2 seconds after the beginning of the reading. (FIG. 4A-1) kinetics of net agonist-promoted BRET signal using data from FIG. 4A-2. Data are representative of 3-4 independent experiments each performed in quadruplicate. FIG. 4B: BRET measured every 0.05 sec for 30 sec in cells expressing β2AR-Rluc and GFP10-Gγ2, and pretreated (open triangle) or not (filled triangle) with 1 μM Iso. ICI (100 μM) was injected 5 seconds after the beginning of the reading. Data are representative of 3-4 independent experiments each performed in quadruplicate. FIG. 4C: BRET measured in HEK293T cells coexpressing β2AR with RLuc-Gβ1 and Gαs-GFP10, and stimulated (▪) or not (□) with 10 μM Iso. Data represent the mean±SEM of 3 independent experiments. *, p<0.05. FIG. 4D: Comparison of long term agonist stimulation on BRET and cAMP production. FIG. 4D-1: BRET measured in cells expressing β2AR-Rluc and GFP10-Gγ2, and stimulated with 10 μM Iso for up to 1 hr. Results are expressed as percentage of the maximum Iso-induced BRET signal obtained at 1 minute. FIG. 4D-2: Iso-stimulated cAMP production measured in membranes derived from β2AR expressing cells, and pretreated or not with 10 μM Iso for the indicated times. Data represent the mean±SEM of 3 independent experiments each performed in triplicate FIG. 4E: BRET measured in the presence of increasing concentrations of Iso in cells expressing β2AR-Rluc and GFP10-Gβ1 (FIG. 4E-1) or GFP10-Gγ2 (FIG. 4E-2), and pretreated (▴) or not (Δ) with 10 μM Iso for 1 hour. Data represent the mean±SEM of 3-4 independent experiments. FIG. 4F: BRET measured in cells expressing α_(2A)AR-Rluc and GFP10-Gγ2, and stimulated with 10 μM UK14304 for up to 1 hr. Results are expressed as percentage of the maximum UK14304-induced BRET signal obtained at 1 min and represent the mean±SEM of 3 independent experiments each performed in duplicate.

FIG. 5: Plasma membrane targeting of CD8-Rluc fusion protein. HEK293T cells were co-transfected with CD8-Rluc or β2AR-Rluc along with HA-GBR2 (used as a plasma membrane indicator). Localization of CD8-Rluc/β2AR-Rluc and HA-GBR2 was assessed by confocal immunofluorescence microscopy, as described below in Material and Methods, and shows colocalization of CD8-Rluc (FIG. 5-2) or β2AR-Rluc (FIG. 5-5) and HA-GBR2 (FIGS. 5-1 and 5-4) at the plasma membrane.

FIG. 6: Agonist-promoted BRET signal between β2AR-Rluc and GFP-Gγ2 is insensitive to pH. BRET was measured in membrane preparations derived from HEK293T cells transfected with β2AR-Rluc and GFP-Gγ2 in the absence (Δ) or presence (▴) of 10 μM Iso. Membranes were prepared as described in Materials and Methods in a lysis buffer at different pH (7.4, 7.0, 6.75, 6.45). Data represent the mean±SEM of three different experiments, each performed in duplicate.

FIG. 7: Vasopressin-selective agonist AVP is unable to modulate BRET signal between β2AR-Rluc/GFP-Gγ2 (FIG. 7-1) or GFP-Gβ1 (FIG. 7-2). HEK293T cells were co-transfected with both β2AR-Rluc, Myc-V2R and either GFP-Gβ1 or GFP-Gγ2, and stimulated or not with 10 μM AVP or Iso. Data represent the mean±SEM of three different experiments, each performed in duplicate and are expressed as percentage of BRET signal obtained in the absence of agonist (basal).

FIG. 8: Interactions between Gβ1 and Gγ2. BRET was measured in HEK293T cells coexpressing Rluc-Gβ1 and GFP-Gγ2 along with HAβ2AR and Gαs, and stimulated (▪) or not (□) with 10 μM Iso. Data represent the mean±SEM of three different experiments, each performed in duplicate.

FIG. 9: BRET signal between β2AR-Rluc/GFP-Gβ1 or GFP-Gγ2 is GTP sensitive. Membrane preparations were obtained from HEK293T cells transfected with β2AR-Rluc and GFP-Gβ1 (FIG. 9-1) or -Gγ2 (FIG. 9-2), and pretreated or not (NT) with the indicated GTP concentrations during 90 min at 25° C. BRET was then measured in the absence (□) or presence (▪) of 10 μM Iso.

FIG. 10: Basal BRET signal between β2AR-Rluc/GFP-Gγ2 is sensitive to a β2AR-inverse agonist. Membrane preparations were obtained from HEK293T cells transfected with β2AR-Rluc and GFP-Gγ2, and stimulated or not (basal) with 10 μM Iso or ICI-118551. Data represent the mean±SEM of three different experiments, each performed in duplicate.

FIG. 11: Kinetics analysis of β2AR/Gγ2 interactions. BRET was measured every second for 1 minute in cells expressing β2AR-Rluc and GFP-Gγ2. Iso (10 μM) was injected 5 seconds after the beginning of the reading. Data are representative of 5 independent experiments each performed in quadruplicate.

FIG. 12: BRET¹, BRET², FRET (FIG. 12-3) titration curves. HEK293T cells were co-transfected with Gα_(i1)-Rluc, GBR1-GFP2 and increasing amounts of GBR2-EYFP. BRET¹ (FIG. 12-1) and BRET² (FIG. 12-2) signals were detected after addition of DeepBlueC or Coelenterazine h, respectively, on the cells. BRET¹ and BRET² were measured using the Fusion-α and the modified TopCount (PerkinElmer), respectively. FRET signal was measured using the Flexstation (Molecular Devices). FIG. 12-4 shows the respective signals measured schematically.

FIG. 13: HEK293T cells will be co-transfected with Gα_(i)122-Rluc, GFP10-Gγ₂ together with α_(2A)AR-eYFP. BRET¹ (detecting interactions between Gα_(i)122-Rluc and α_(2A)AR-eYFP) and BRET² (detecting interactions between Gα_(i)122-Rluc and GFP10-Gγ₂) signals were detected after addition of DeepBlueC or Coelenterazine h, respectively, on the cells. BRET¹ (FIG. 13-2) and BRET² (FIG. 13-3) signals were measured using the Fusion-α and the modified TopCount (PerkinElmer), respectively. FRET signal (FIG. 13-4) (detecting interactions between GFP10-Gγ₂ and α_(2A)AR-eYFP) was measured using the Flexstation (Molecular Devices). FIG. 13-1 shows the respective signals measured schematically.

FIG. 14: BRET was measured in HEK293T cells co-expressing β₂AR-GFP and either Rluc-Gβ₁ (FIG. 14-1) or Rluc-Gγ₂ (FIG. 14-2), and stimulated (▪) or not (□) with 10 μM Iso.

FIG. 15: FIG. 15A: Schematic representation of receptor and G protein subunits BRET constructs. Receptors were fused at their C-terminal with Rluc or GFP variants (GFP10; YFP; Venus). Human Gβ₁ and Gγ₂ G protein subunits were both fused at their N-terminal to Rluc or GFP10 while Rluc was inserted between L91 and K92 residues (Gα_(i1)-91Rluc) or between residues E122 and L123 (α_(i1)-122Rluc) within Gα_(i1) subunit. FIG. 15B: Overall architecture of Gα modified from Weng et al.³⁹, consisting of three major domains (the GTPase domain, the helical domain (connected by linker 1) and the N-terminal domain) and three switch regions (switch I, II, and III). Arrows indicate the different positions of the two main Rluc probes used in the study.

FIG. 16: View of the structures of Gαi1 (FIG. 16-1), Gαi1-91Rluc (FIG. 16-2) and Gαi1-122Rluc (FIG. 16-3). Schematic representation of Gα_(i1) structure (light blue; PDB code 1GG2) fused or not to luciferase (Dark blue; PBD code 1LC1) in different positions within the protein, as indicated. The flexible linker, SGGGGS, used to fuse luciferase is shown in green. Structures were visualized by means of the software YASARA.

FIG. 17: Plasma membrane targeting of Gαi1-91Rluc (FIG. 17-2) and Gα_(i1)-122Rluc (FIG. 17-5) fusion proteins. HEK293T cells were co-transfected with either Gα_(i1)-91Rluc or Gα_(i1)-122Rluc along with HA-GBR2 (used as a plasma membrane indicator)(FIGS. 17-1 and 17-4). Localization of Gα_(i1)-91Rluc or Gα_(i1)-122Rluc and HA-GBR2 was assessed by confocal immunofluorescence microscopy, as described in material and methods and shows co-localization of Gα_(i1)-91Rluc or Gα_(i1)-122Rluc and HA-GBR2 at the plasma membrane.

FIG. 18: Functionality of Gαi1-91Rluc and Gαi1-122Rluc fusion proteins. Inhibition of cAMP production in HEK293T cells expressing α_(2A)AR-eYFP and either Gαi1, Gαi1-91Rluc or Gαi1-122Rluc was measured in the presence of increasing concentrations of UK14,304. Data are expressed as percentage decrease of cAMP level relative to control and represent the mean±S.E.M. of 4 independent experiments. No inhibition of the cAMP production could be observed in the absence of co-transfected Gαi.

FIG. 19: Configurations of the different BRET assays used to probe receptor-mediated G protein activation. Schematic representation of a GPCR (purple, Rhodopsin PDB code 1L9H) and a heterotrimeric G protein composed of αi1 (FIG. 19-A19-F), β1 (FIG. 19A), and γ2 subunits (FIG. 19B) (light blue, red and yellow respectively; PDB code 1GG2) interacting at the plasma membrane, fused to luciferase (blue; PBD code 1LC1) or to GFP (green; PDB code 1GFL), as indicated.

FIG. 20: BRET measurements of GPCRs and Gα_(i1)β₁γ₂ interactions in living cells. BRET was measured in cells coexpressing Rluc-tagged α_(2A)AR (α_(2A)AR-Rluc) with either GFP10-Gβ₁ or GFP10-Gγ₂ (FIGS. 20-1, 20-2 and 20-3) or in cells coexpressing GFP2-tagged α_(2A)AR (α_(2A)AR-GFP2) with either Gα_(i1)-91Rluc or Gα_(i1)-122Rluc (FIGS. 20-4, 20-5 and 20-6), and stimulated (black) or not (white) with 10 μM UK14,304. Data represent the mean±S.E.M. of 3-4 independent experiments. *, P<0.05.

FIG. 21: Pertussis toxin-sensitivity of receptor-mediated G protein activation. BRET measured in the absence or presence of UK14,304 in cells coexpressing α2AAR-Rluc in the presence of either GFP10-Gβ1 or GFP10-Gγ2, or in cells expressing Gαi1-91Rluc or Gαi1-122Rluc with either α2AAR-GFP2 or GFP10-Gγ2, as indicated, and pretreated (PTX) or not (NT) with pertussis toxin during 16 h at 37° C. Results are expressed as the difference in BRET signal observed in the presence and absence of ligand and represent the mean±s.e.m. of 4 independent experiments, each performed in duplicate.

FIG. 22: BRET measured every 0.05 s for 12 s in cells expressing α_(2A)AR-Venus with either Rluc-Gγ₂ (FIG. 22-1), or Gα_(i1)-91Rluc (FIG. 22-2) or Gα_(i1)-122Rluc (FIG. 22-3). UK14,304 (10 μM) was injected 2 s after the beginning of the readings. Data are representative of 3-4 experiments, each performed in quadruplicate.

FIG. 23: BRET measured in cells coexpressing Gα_(i1)-91Rluc or Gα_(i1)-122Rluc with either GFP10-tagged β₂-adrenergic receptor (β₂AR-GFP10) (FIGS. 23A-1 and 23A-2) or YFP-tagged Calcitonin receptor like receptor (CGRP-R=CRLR-YFP+RAMP1) (FIGS. 23B-1 and 23B-2), and stimulated (black) or not (white) with their respective selective agonists (Isoproterenol [Iso] and CGRP, 10 μM). *, P<0.05.

FIG. 24: BRET measurements of α2BAR and Gαi1 interaction in living cells. BRET measured in cells coexpressing Gαi1-122Rluc and α2BAR-Rluc, and stimulated (black) or not (white) with 10 μM Dexmedetomidine. Data represent the mean±s.e.m. of 3 independent experiments. *, P<0.05.

FIG. 25: BRET measured as in FIG. 20 in the presence of selective α_(2A)-adrenergic ligands (UK14,304 [UK], Dexmedetomidine [Dex], Clonidine [Clo], Yohimbine [Yo], RX821002 [RX], 10 μM). Results are expressed as the difference in BRET signal observed in the presence and absence of ligand. *, P<0.05; ***, P<0.001 compared with UK-promoted BRET with Gβ1 (FIG. 25-1) Gγ₂ (FIG. 25-2) Gα_(i1)-91 (FIG. 25-3), and Gα_(i1)-122 (FIG. 25-4).

FIG. 26: Assessment of the dynamic nature of receptor/Gα_(i1)β₁γ₂ interactions. (a-c) BRET titration curves. BRET was measured in cells expressing a fixed amount of the indicated Rluc-tagged constructs and increasing amounts of the indicated GFP-tagged protein, and treated (▴) or not (Δ) with 10 μM UK14,304 (α_(2A)AR) (FIGS. 26A-1-4) or Iso (β₂AR) (FIGS. 26B-1-2, FIG. 26C). GFP/Rluc ratios leading to 50% of the maximal BRET (BRET₅₀) are presented in the tables. (d) Western blots illustrating the co-immunoprecipitation of β₂AR-Rluc with both GFP10-Gβ₁ (FIG. 26D-1) or GFP10-Gγ₂ (FIG. 26D-2). Co-immunoprecipitations were carried out in the presence or absence of 10 μM Iso. In parallel, control experiments were carried out in cells transfected only with GFP10-Gβ₁ or GFP10-Gγ₂. Data shown are representative of 3 independent experiments.

FIG. 27: BRET measurements of Gα_(i1)β₁γ₂ subunits interactions in living cells. FIG. 27A: BRET measured in cells coexpressing either Gα_(i1)-91Rluc (FIG. 27A-2) or Gα_(i1)-122Rluc (FIG. 27A-3) with GFP10-Gγ₂ and α_(2A)AR, and stimulated (black) or not (white) with 10 μM UK14,304. Data represent the mean±s.e.m. of 3-4 independent experiments. *, P<0.05. Shown schematically in FIG. 27A-1. FIG. 27B: Cells cotransfected with Gα _(i1)-91Rluc, GFP10-Gγ₂ and α_(2A)AR-Venus were stimulated (black) or not (white) with 10 μM UK14,304. BRET (FIG. 27B-2) or BRET (FIG. 27B-3) was then measured by adding Coelenterazine h or DeepBlueC coelenterazine, respectively. Data represent the mean±s.e.m. of 4 independent experiments. *, P<0.05. Shown schematically in FIG. 27B-1. FIG. 27C: BRET measured as in FIG. 27A in the presence of α₂-adrenergic selective ligands (UK14,304 [UK], Dexmedetomidine [Dex], Clonidine [Clo], Yohimbine [Yo] and RX821002 [RX], 10 μM). Results are expressed as the difference in BRET signal observed in the presence and absence of ligand. *, P<0.05; ***, P<0.001 compared with UK-promoted BRET, respectively as FIG. 27C-1 and FIG. 27C-2.

FIG. 28: Insight into Gα_(i1)β₁γ₂ structural rearrangements. BRET measured in cells coexpressing either Gα_(i1)-91Rluc (FIG. 28A-1), Gα_(i1)-122Rluc (FIG. 28A-2) or Gα_(i1)-60Rluc (FIG. 28B) with GFP10-Gγ₂, in the presence of different GPCRs (α_(2A)-, β₁- and β₂AR, dopamine-D1, δ-opioid [δOR], muscarinic-M2 [m2], chemokine-CXCR4 and -CCR5, calcitonin gene related peptide [CGRP-R=CRLR+RAMP1], prostaglandine-EP4, vasoactive intestinal peptide [VIP] and secretine). Results are expressed as the difference in BRET signal observed in the presence and absence of ligand and represent the mean±s.e.m. of 3-4 independent experiments.

FIG. 29: Kinetic analysis of receptor and QL-Gαi1 mutant interaction. BRET measured in cells coexpressing QL-Gαi1-122Rluc and β2AR-GFP10 and stimulated or not with 10 μM Iso for up to 4 min. Results are expressed as the difference in the BRET signal observed in the presence and absence of ligand and represent the mean±s.e.m. of 2 independent experiments.

FIG. 30: BRET measurements of GPCRs and Gas interactions. BRET was measured in cells coexpressing Gαs-71-GFP10 with Rluc-Gγ2 in the presence of different GPCRs (β1- and β2adrenergic, dopamine-D1, vasopressin-V2, prostaglandine-EP4, vasoactive intestinal peptide [VIP] and secretine). Results are expressed as the difference in BRET signal observed in the presence and absence of ligand and represent the mean±s.e.m. of 3 independent experiments.

FIG. 31: FIG. 31A: Schematic complex in the plasma membrane between rhodopsin (gray; PDB code 1GZM) and the inactive heterotrimeric G protein composed of α_(i1), β₁, and γ₂ subunits (light blue/violet, red and yellow, respectively; PDB code 1GG2). Gα_(i1) N-terminal helix (Nα) is shown in brown, while Gα_(i1)-GTPase and Gα_(i1)-helical domains (α_(i1)H) are in light blue and violet respectively. Linker 1 connecting Gα_(i1)-GTPase to the Gα_(i1)H is represented in green. Both Gα_(i1)N (Nα) and Gγ₂ C-terminal helix (Cγ) are anchored to the membrane trough lipid modification. Arrows highlight the different positions of Rluc or GFP probes inserted in the Gα_(i1)β₁γ₂ heterotrimer. FIG. 31B: Schematic representation of structural rearrangement within Gα_(i1)β₁γ₂ detected by BRET following receptor activation. Rluc probes within Gα_(i1) are shown in blue while GFP probe at the N-terminal of Gγ₂ is shown in green. The scheme represents an opening of Gα_(i1)-GTPase and Gα_(i1)H trough linker 1 (like a clamp), thus increasing RLuc91-Gγ₂N and RLuc122-Gγ₂N distances while shortening that of RLuc60-Gγ₂N. These rearrangements would thus create an exit route for the guanine nucleotide.

FIG. 32: Schematic representation of the complexes for the possible biosensors presented in the patent. FIG. 32A: BRET assay between a GPCR and one of the heterotrimeric G proteins, either Gail (at position 60, 91 or 122), Gβ1 or Gγ2, FIGS. 32A-1, 32A-2, and 32A-3, respectively. FIG. 32B: BRET assay between Gαi1 (at position 60, 91 or 122) and either Gβ1 or Gγ2. FIGS. 32B-1, 32B-2, and 32B-3, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Definitions

Unless specifically defined, the terms used in the present application have the meanings that one of ordinary skill in the art would ascribe to them.

In order to provide a clear and consistent understanding of the terms used in the present specification, a number of definitions are provided below. Moreover, the present description refers to a number of routinely used chemical and technical terms; definitions of selected terms are provided for clarity and consistency.

The use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims and/or the specification may mean “one”, but it is also consistent with the meaning of “one or more”, “at least one”, and “one or more than one”. Similarly, the word “another” may mean at least a second or more.

As used in this specification and claim(s), the words “comprising” (and any form of comprising, such as “comprise” and “comprises”), “having” (and any form of having, such as “have” and “has”), “including” (and any form of including, such as “include” and “includes”) or “containing” (and any form of containing, such as “contain” and “contains”), are inclusive or open-ended and do not exclude additional, unrecited elements or method steps.

The term “about” is used to indicate that a value includes an inherent variation of error for the device or the method being employed to determine the value.

GPCR G protein-coupled receptor GDP Guanosine diphosphate GTP Guanosine triphosphate BRET Bioluminescence resonance energy transfer FRET Fluorescence resonance energy transfer Biosensor Type of biomolecular probe that measures the presence or concentration of biological molecules, biological structures, etc., by translating a biochemical interaction at the probe surface into a quantifiable physical signal such as light, electric pulse or fluorescent signal. Ligand texture Ligand-selective receptor conformations fingerprint introduce the concept of ‘texture’ to drug effects, with respect to ligands possessing quality in addition to quantity of efficacy.³³ β₂AR beta 2-adrenergic receptor Rluc Renilla luciferase GFP Green fluorescent protein Iso Isoproterenol Feno fenoterol ICI ICI-118551 V2R V2-vasopressin α_(2A)AR alpha 2-adrenergic receptor SNSR-4 Sensory neuron-specific receptor 4 Ip Immunoprecipitation Ib Immunoblot Materials and Methods cDNA Expression Vectors

All receptor constructs were fused in frame at their carboxyl terminus to either the humanized Rluc, or GFP10, or YFP/Venus. GFP10 is a variant form of the green fluorescent protein (GFP) previously reported²³. Plasmids encoding β2AR-Rluc and V2R-Rluc were described previously^(23,58). β2AR-D79N-Rluc construct was generated using oligonucleotide-directed mutagenesis (Quick-Change™ Site-directed Mutagenesis Kit, Stratagene) and pcDNA3.1-β2AR-Rluc as template. α2AAR-Rluc and TPα-Rluc were obtained by subcloning the α2AAR and TPα receptors coding sequences lacking their stop codon into the humanized Rluc-N1 and -N3 vectors, respectively (Perkin Elmer, Lifescience). Plasmid encoding SNSR4-Rluc (pcDNA3.1-SNSR4-Rluc) was a gift from Astra-Zeneca, Montréal.

To obtain α_(2A)AR-Venus, the coding sequence of Venus³⁵ was PCR amplified and cloned in frame with the C-terminal of α_(2A)AR by replacing the Rluc tag in the pα_(2A)AR-Rluc vector. α_(2A)AR-GFP2 and α_(2B)AR-GFP2 were obtained by subcloning the receptor coding sequences lacking their stop codon into the humanized pGFP2-N1 vector (Perkin Elmer, Lifescience).

All Gβ and Gγ constructs were fused in frame at their amino-terminus to the humanized Rluc or GFP10. Plasmids encoding GFP10-Gβ1 and -Gγ2 were a gift from Biosignal/Perkin Elmer, Lifescience. Gαs-GFP10 was generated by subcloning GFP10 into EcoRI/BsrGI sites of the Gαs-EGFP construct generously provided by Dr. Rasenick⁴³.

Coding sequence of humanized Rluc (PerkinElmer, Lifescience) was PCR amplified without its STOP and inserted via flexible linkers (SGGGGS) in the coding sequence of human Gα_(i1) between: residues L91 and K92 (Gα_(i1)-91Rluc) or residues E122 and L123 (Gα_(i1)-122Rluc) or residues G60 and Y61 (Gα_(i1)-60Rluc). Similar strategy was used to obtained the Rluc-tagged constitutively active Q204L-Gα_(i1) mutant (QL-Gα_(i1)-122Rluc) but using Q204L-Gα_(i1) as a template.

Plasmid encoding Rluc-Gγ₂ was obtained by subcloning the human Gγ₂ coding sequence coding sequence into the humanized Rluc-N1 vector (Perkin Elmer, Lifescience).

To generate a CD8-Rluc construct, the fragment of pcDNA-CD8-βARK-C18 encoding the extracellular and transmembrane domain of the CD8 lymphocyte-specific receptor (from codon 1 to 209) was subcloned into the humanized pRluc-N1 vector (Perkin Elmer, Lifescience). The resulting DNA construct, CD8-Rluc, expresses the extracellular and transmembrane domain of CD8 fused to Rluc at its carboxyl tail (intracellular domain).

Ramp1 and N-terminal tagged-HA-GABAB-R2 (GBR2) were a gift from GlaxoSmithkline and has been previously described⁵⁹. The plasmid encoding N-terminal Myc-tagged vasopressin type-2 receptor (Myc-V2R) has also been described previously⁶⁰. Vector encoding CRLR-YFP was a generous gift from Patrick Sexton.

Plasmid encoding the βarrestine1 truncated form, YFP-βarr1-T383, was a generous gift from Stephane Laporte²⁵.

All plasmids encoding wild type G protein subunits (Gαs, Gαi1, Gαi2, Gαq, Gα11, Gα13, Gβ1, Gγ2) were obtained from the Güthrie Research Institute (PA).

All generated constructs were confirmed by sequencing.

Cell Culture and Transfections

Human embryonic kidney 293 cells (HEK293T) were cultured in Dulbecco's modified Eagle's medium supplemented with 10% fetal bovine serum, 2 mM L-glutamine, 100 unit/ml penicillin/streptomycin at 37° C. in a humidified atmosphere at 95% air and 5% CO2. In all cases, transient transfections were performed 24 hours after cell seeding using the calcium phosphate precipitation method, except for the immunofluorescence studies where FuGENE6 (Roche Molecular Biochemicals) was utilized according to the manufacturer's protocol.

Immunofluorescence Confocal Microscopy

HEK293T cells were seeded and transfected in 6 well plates containing glass coverslips precoated with 1 mg/ml poly-L-Lysine. For CD8-Rluc or β2AR-Rluc and HA-GBR2 localization experiments, 48 hrs after transfection, HEK293T cells were incubated overnight at 4° C. with rat monoclonal anti-HA antibody (3F10) for cell surface immunostaining of GBR2. HA-labelling was revealed using a Alexa 488-conjugated goat anti-rat antibody (Molecular Probes) for 30 min at RT. Cells were then washed, fixed with 3% paraformaldehyde in PBS for 15 min, permeabilized for 10 min with 0.3% Triton X-100 in blocking buffer (PBS-0.2% BSA), and incubated for 30 min with a mouse anti-Renilla luciferase antibody (Chemicon International). Immunoreactivity was revealed using a Texas Red-conjugated secondary goat anti-mouse antibody (Molecular Probes). Images were acquired on a Leica TCS SP1 laser-scanning microscope.

Cell Membranes Preparation

Cell membranes used for the measurement of the adenylyl cyclase activity or the radioligand binding assay were prepared as previously described⁶¹.

Adenylyl Cyclase Activity

Adenylyl cyclase activity was determined in membrane preparation (FIG. 4 c) or in whole cells (FIGS. 3 b, d and e). Membrane adenylyl cyclase activity was determined using anion exchange chromatography, as previously described⁶¹. For whole cell cAMP production, cells were detached in PBS-5 mM EDTA and resuspended in PBS-0.1% glucose at RT and incubated at RT for 10 min in the presence of 0.7 mM 3-isobutyl-1-methylxanthine (IBMX). The indicated drugs were then added for 30 min at 37° C. and the reaction stopped by adding lysis buffer (pH 7.3) for 10 min at RT according to Molecular Devices Corporation. cAmp content of lysated cells was then measured using a fluorescent based-competitive immunoassay (Catchpoint™ cyclic-AMP Fluorescent Assay Kit-96-well format, Molecular Devices Corporation). Fluorescence readings were performed using the FlexStation™ instrument (Molecular Devices Corporation).

Radioligand Binding Assay

β2AR-Rluc or D79N-β2AR-Rluc binding properties were determined as previously described⁶¹ by competitive binding of 30 pM ¹²⁵I-cyanopindolol (CYP) (NEN, Perkin Elmer) by increasing concentrations of unlabeled isoproterenol. Nonspecific binding was estimated in the presence of 10 μM unlabeled cyanopindolol.

Bioluminescence Resonance Energy Transfer (BRET) Assay

BRET² was used as previously described²³.

Rluc-tagged receptors and G protein constructs were transiently co-transfected in HEK293T cells. Except when otherwise specified, all BRET measurements were made in cells coexpressing receptor-Rluc and either Gαs-GFP10, GFP10-Gβ1 or GFP10-Gγ2 along with their complementary subunits (Gβ1γ2, Gγ2αs or Gβ1αs, respectively). For the experiment using α2AAR-Rluc (FIG. 4 f), the G protein heterotrimer was complimented with the most specific Gαi2. Forty-eight hours post-transfection, cells were washed twice with PBS, detached with PBS-5 mM EDTA and resuspended in PBS-0.1% glucose at RT. Cells (50 μg of proteins per well) were then distributed in a 96-well microplate (white Optiplate, PerkinElmer) and incubated in the presence or absence of different ligands for 1 min except in the case of the kinetic studies where the times are indicated. DeepBlueC™ coelenterazine (PerkinElmer) was added at a final concentration of 5 μM, and readings (except for the kinetic studies; see below) were collected using a modified top-count apparatus (TopCount.NX™, Packard Bioscience) that allows the sequential integration of the signals detected in the 370-450 nm and 500-530 nm windows using filters with the appropriate band pass (Chroma). The BRET signal was determined by calculating the ratio of the light emitted by GFP10 (500-530 nm) over the light emitted by the Rluc (370-450 nm). The net BRET values were obtained by subtracting the BRET background signal detected when the Rluc-tagged construct was expressed alone from BRET signals detected in cells coexpressing both Rluc- and GFP10-tagged constructs. Identical background values of 0.15 were obtained for all Rluc-tagged constructs expressed alone. The expression level of each protein was determined by direct measurement of total fluorescence and luminescence on aliquots of the transfected cells. The GFP10 total fluorescence was measured using a FluoroCount (PerkinElmer) with an excitation filter at 400 nm, an emission at 510 nm, and the following parameters: gain 1; PMT 1100 V; time 1.0 s. After fluorescence measurement, the same cells were incubated for 8 min with Coelenterazine h (Molecular Probes) at a final concentration of 5 μM and the total luminescence of cells was measured using a LumiCount (PerkinElmer Life Sciences) with the following parameters: gain 1; PMT 900 V; time 0.5 s. In contrast to DeepBlueC coelenterazine, Coelenterazine h does not lead to energy transfer between Rluc and GFP10 and thus allows the assessment of the total Rluc activity. To avoid variations in the BRET signal that could result from fluctuations in the relative expression levels of the energy donor and acceptor, transfection conditions were designed so as to maintain a constant GFP10/Rluc expression ratio in each experimental set.

For titration experiments (FIG. 26 a-c), the expression level of each tagged-protein was determined by direct measurement of total fluorescence and luminescence on aliquots of the transfected cells. Total fluorescence was measured using a FluoroCount (PerkinElmer) with an excitation filter at 400 or 485 nm and an emission filter at 510 or 530 nm in the case of GFP2/GFP10 or YFP/Venus, respectively, and the following parameters: gain 1; PMT 1100 V; time 1.0 s. After fluorescence measurement, the same cells sample was incubated for 8 min with coelenterazine h (Molecular Probes) at a final concentration of 5 μM and the total luminescence of cells was measured using a LumiCount (PerkinElmer Life Sciences) with the following parameters: gain 1; PMT 900 V; time 0.5 s.

For kinetic analysis of receptor/Gαβγ interactions, DeepBlueC™ or Coelenterazine h luciferase substrate was added prior to the injection of the different ligands using the Mithras LB 940 apparatus (Berthold) and MicroWin2000 software. Readings were then collected at 0.05 or 0.1 sec intervals. Injection of the different ligands was included with in the kinetic program to allow a baseline recording followed by real-time recording of the BRET changes. The BRET signals were determined for each time by calculating the ratio of the light emitted by GFP over that emitted by the Rluc. To determine the half-time (t½) of Iso-induced BRET, data were represented as the difference between the Iso-induced BRET signals and the average of basal BRET signal (net agonist-promoted BRET signal). Curves were fitted using a non-linear regression and one phase exponential association fit equation (GraphPad Prism).

For kinetic analysis of QL-Gα_(i1)-122Rluc/α₂AR-GFP10 interactions (FIG. 24), DeepBlueC™ coelenterazine was added to the cells prior to agonist addition and readings were collected at 24 s intervals using the modified TopCount.NXT™ instrument.

For agonist stimulations longer than 1 minute, cells were first treated with the ligand (agonist, antagonist, inverse agonist) and DeepBlueC™ coelenterazine added immediately before BRET readings in the modified TopCount.NXT™ apparatus. For these experiments, readings were taken directly in 96 well plates 72 hours post-transfection without detaching the cells.

Immunoprecipitation of Receptor/Gβγ Complexes

HEK293T cells were cotransfected in 100-mm plates with plasmids encoding either GFP10-Gβ₁ or -Gγ₂ in the presence or absence of α₂AR-Rluc. Forty-eight hours after transfection, cells were stimulated or not with 10 μM Iso for 30 sec and proceeded as previously described³⁷. Immunoprecipitation was performed using the anti-α₂AR polyclonal antibody (SantaCruz). Immune complexes were eluted with Laemmli buffer containing 1 M urea and 50 mM dithiotreitol for 15 min at 45° C. Immunoblotting of α₂AR-Rluc was performed using a mouse anti-Rluc (ChemiconInternational) while immunoblotting of GFP10-Gβ₁ or -Gγ₂ were performed using a monoclonal anti-GFP (Clontech). Immune complexes were then visualized by chemiluminescence detection using anti-mouse horseradish peroxidase-conjugated IgG.

Statistical Analysis

One-way ANOVA followed by Student's t test was used (*) to determine statistically significant differences. When indicated, one-way ANOVA followed by Dunnett's test (#) was used as supplementary analysis to determine statistically significant differences from indicated control.

BRET Measurements Between Receptors and Gαβγ Fusion Proteins

The prototypical family 1 GPCR, β2-adrenergic receptor (β2AR), was used as the main model to probe the interaction with its preferred Gα subunit, Gαs, and the ubiquitous Gβ1 and Gγ2 subunits following receptor activation. For this purpose, a proximity-based BRET² assay was developed (FIG. 1) that relies on the non-radiative transfer of energy between the energy donor Renilla reniformis luciferase (Rluc) and a blue shifted variant of the Aequorea Victoria green fluorescent protein known as GFP10 that serves as the energy acceptor²³. The receptor/G protein interaction assay was conceived by fusing Rluc to the carboxyl terminus of the β2AR while GFP10 was covalently attached to either Gαs, Gβ1 or Gγ2. For Gαs, the fluorophore was inserted within the linker 1 region between the helical and GTPase domains, as previously described⁴³, while the Gβ1 and Gγ2 subunits were fused to GFP10 at their amino terminus. Also as previously described²³, binding and signaling properties of the β2AR-Rluc were identical to those of the wild-type receptor. In agreement with previous reports^(44,43), the fusion of GFP10 to Gαs, Gβ1 and Gγ2 was well tolerated, the fusion proteins being well targeted and active at the plasma membrane (data not shown).

Receptor/G protein interactions were measured in living HEK293T cells co-expressing β2ARRluc and either Gαs-GFP10, GFP10-Gβ1 or GFP10-Gγ2 in combination or not with the complementary untagged G protein subunits (FIG. 2 a). The same kind of assay was performed in HEK293T cells co-expressing RLuc-Gβ1 or RLuc-Gγ2 in combination with untagged Gαs with either β2ARGFP10 (FIG. 14). As shown in FIG. 2 a, when GFP10-tagged G protein subunits were expressed individually with β2AR-Rluc, basal BRET signals were observed in all cases most likely reflecting constitutive interactions between the receptor and the G protein heterotrimer under basal conditions (see below). Exposure to the β-adrenergic agonist, isoproterenol (Iso), induced a modest but significant increase of BRET over the basal signal in cells co-expressing the β2AR-Rluc and either GFP10-Gβ1 (+36%) or GFP10-Gγ2 (+15%). Likewise, this agonist-modulated BRET signal was detected for the direct interaction between Gα_(i1)-Rluc and β₂AR-GFP10 in the absence of Gβ and Gγ subunits complementation, but not Gαs-GFP10 (FIG. 2 c). This indicates that, in the absence of co-expressed complementary subunits, the agonist-promoted engagement of the G protein by the receptor could be detected by monitoring the BRET between the receptor and either Gβ1, Gγ2 or Gαi.

Co-expression of untagged complementary subunits significantly improved the sensitivity of the system to detect the agonist-promoted G protein engagement (FIG. 2 a). In the case of Gαs-GFP10, co-expression of untagged Gβ1γ2 dimer (Gαs-GFP10+Gγ2+Gβ1) allowed the detection of an agonist-promoted increase in BRET with β2AR-Rluc (+68%). Similarly, for both GFP10-Gβ1 and GFP10-Gγ2, transfection with their complementary subunits (GFP10-Gβ1+Gγ2+Gαs or GFP10-Gγ2+Gβ1+Gαs) greatly favored the engagement of Gβγ by the receptor, as reflected by the substantial enhancement of the agonist-promoted BRET signal (+125% vs +36% with GFP10-Gβ1 and +63% vs +15% with GFP10-Gγ2). This potentiation most likely reflects the requirement for a stoichiometric expression of the three subunits for their proper processing and targeting to the plasma membrane⁴⁵ (data not shown).

Heterotrimer formation and trafficking of the Gβγ dimer to the cell surface is not sufficient to confer an agonist-stimulated BRET signal. Indeed, although all Gα subunits can promote the proper targeting of Gβ1 and Gγ2 to the cell surface⁴⁵ (data not shown), they showed significant selectivity in their ability to favor the agonist-stimulated BRET between the receptor-Rluc and GFP10-Gβ1 or GFP10-Gγ2. For instance, Gαs and to a lesser extent Gαi but not Gαq or Gα11 potentiated the agonist-induced BRET signal, despite similar expression levels of BRET donors and acceptors in each condition (FIG. 2 a). This is consistent with the rank order of selectivity for various Gα subunits previously reported for the β2AR: Gs>Gi>>>Gq=G11⁴⁶. The importance of the Gα subunit in directing the selectivity of Gβγ engagement was also observed for Rluc-tagged V2-vasopressin (V2R), α2A-adrenergic (α2AAR), sensory neuron-specific (SNSR-4) and thromboxane A2 receptor (TPαR) for which the agonist-stimulated BRET with GFP10-β1 and GFP10-Gγ2 was only potentiated by specific Gα subsets (FIG. 2 b). V2R and α2AAR showed a very high level of selectivity, only one Gα subunit (Gαs and Gαi2 for V2R and α2AAR, respectively) potentiating the BRET response. For SNSR, both Gαq and Gαi2 potentiated the response, whereas for TPαR, the presence of Gα13 or Gαq, and to a lower extent Gαs or Gαi2 amplified the agonist-promoted BRET. In all cases, the Gα selectivity inferred indirectly through the receptor/βγ BRET assay reflected well the coupling specificity generally attributed to these receptors by functional assays^(47,48).

This data therefore demonstrates that the identity of the Gα subunit plays a central role in determining the selectivity of interaction between receptors and specific Gβγ pairs. The data further demonstrates that the receptor-β/γ BRET-based assay can be advantageously used to probe the selectivity of interaction between receptors and the Gα subunits in living cells using a unique assay mode.

To confirm the specificity of the BRET signal observed between GFP10-Gβ1 or GFP10-Gγ2 and the receptor-Rluc, a negative control consisting of a truncated form of CD818 fused to Rluc (CD8-Rluc) was generated. This construct exhibits a subcellular distribution similar to that of the β2AR-Rluc (FIG. 5) and similarly exposes the Rluc moiety to the inner face of the plasma membrane. As shown in FIG. 2 d, co-expression of CD8-Rluc with GFP10-Gβ1 or GFP10-Gγ2 in the presence of their complementary G protein subunits (GFP10-Gβ1+Gγ2+Gαs or GFP10-Gγ2+Gβ1+Gαs) only led to marginal basal BRET signals that were not modulated by agonist stimulation. This contrasted with the robust basal and agonist-promoted BRET signals observed between β2AR-Rluc and GFP10-Gβ1 or GFP10-Gγ2 for comparable Rluc and GFP10 expression levels, thus confirming the specificity of the BRET signals detected between the receptors and Gβγ subunits.

Agonist-Modulated BRET Signals Reflect Receptor-Mediated G Protein Activation

Agonist stimulation led to a concentration-dependent elevation in the BRET between β2AR-RLuc and either GFP10-Gβ1 or -Gγ2 (FIG. 3 a), which paralleled the rise in cAMP production (FIG. 3 b), indicating that it faithfully reflects the activation state of the receptor. The intrinsic efficacies of different β-adrenergic agonists to activate the adenylyl cyclase (FIG. 3 d) were also faithfully reflected in the ligand-promoted BRET responses detected between β2AR-Rluc and the GFP10-G protein subunits. Indeed, the partial agonist fenoterol (Feno) promoted only a fraction of the response evoked by the full agonist isoproterenol while the inverse agonist ICI-118551 (ICI) completely blocked the agonist-stimulated response (FIG. 3 c). This is also true when considering the interaction between Gα_(i1)-Rluc and α_(2A)AR-GFP10 (FIG. 3 e) for which the partial agonists Clonidine and Dexometodine (Dex), induced only a fraction of the full agonist modulation UK14304, while preincubation with the antagonist RX821002 completely blocked the agonist-stimulated response. Both α₂ adrenergic antagonists RX821002 and Yohimbine are unable to modulate the BRET signal between the α₂ receptor and the Gα_(i1) subunit to exclude the possibility that the observed changes in BRET reflect local variations in pH that could modify the properties of Rluc and/or GFP10, additional measurements were carried out in buffered membrane fractions rather than in whole cells. Identical agonist-induced increases in BRET between β2AR-Rluc and GFP10-Gγ2 were detected for all pH values tested (6.5 to 7.5) (FIG. 6). Non-specific cellular changes resulting from GPCR activation were also ruled out since stimulation of another untagged Gαs-coupled receptor, the V2R, did not promote any increase in BRET between β2AR-Rluc and either GFP10-Gβ1 or -Gγ2 subunits (FIG. 7).

Finally, the specificity of the agonist-stimulated signal is further supported by the observation that isoproterenol did not promote any change in the strong BRET signal observed for the obligatory dimer between Rluc-Gβ1 and GFP10-Gγ2 (FIG. 8). The BRET-detected engagement of Gβγ following agonist binding appears to be a true reflection of the conformational changes linked to receptor activation. Indeed, mutation of aspartate 79 by an asparagine (D79N-β2AR-Rluc), which disrupts β2AR mediated-adenylyl cyclase activation without preventing agonist binding⁵⁰ (FIG. 3 f, g), almost completely abolished the agonist-promoted BRET between the β2AR and either GFP10-Gβ1 or GFP10-Gγ2 (FIG. 3 h).

Interestingly, the basal BRET signal detected between the activation deficient D79N-β2AR-Rluc and GFP10-Gβ1 or GFP10-Gγ2 was lower than that observed for the wild type receptor, indicating that it reflects, at least in part, a receptor/G protein coupling resulting from receptor constitutive activity⁵¹. Also, consistent with the notion that the agonist-promoted increase in BRET reflects receptor-mediated G protein activation is the observation that the non hydrolysable GTP analog GTPγS, which interrupts the activation/inactivation cycle of the G protein, attenuated the agonist-promoted signal (FIG. 3 i). In addition to its effect on the agonist stimulated BRET, the nucleotide also reduced the basal BRET observed in the absence of agonist, reinforcing the notion that the basal BRET signal most likely reflects constitutive receptor-G protein interactions. Similar results were obtained when using high concentrations of the hydrolysable nucleotide GTP (FIG. 9). The β2AR-specific inverse agonist, ICI-118551 (ICI) was also found to significantly lower the basal BRET signal between β2AR-Rluc and GFP10-Gγ2 (FIG. 10), thus confirming that the basal BRET signal reflected constitutive receptor activity.

BRET Kinetics Reveal Millisecond Time-Scale G Protein Activation Followed by a Slower Desensitization

The kinetics of Gβγ engagement by the receptor following agonist stimulation was then monitored using real time BRET measurements. As shown in FIG. 4 a, the maximal increase in BRET between β2AR-Rluc and GFP10-Gβ or -Gγ occurred within the first second (t½˜300 msec) after agonist addition. These kinetics are consistent with the very fast GPCR conformational activation switch determined by FRET in living cells¹⁹. The elevated BRET signal remained constant for at least 1 minute in the continued presence of the agonist (FIG. 11). However, the signal rapidly returned to basal values following the addition of the inverse agonist ICI-118551, indicating that the active conformation of the receptor is essential for the sustained engagement of Gβγ (FIG. 4 b). The persistent BRET signal observed in the presence of an agonist can be reconciled with the classical view that a rapid dissociation of Gα and Gβγ from the receptor follows the initial recruitment of the heterotrimer by suggesting that the agonist triggers a new equilibrium between Gβγ-associated and -dissociated β2AR that results in a new steady state where the elevated BRET signal reflects a greater proportion of receptor being associated with Gβγ in the continued presence of agonist. Alternatively, the data may indicate that βγ does not readily dissociate from the receptor following activation.

To further explore the dynamics between Gαs and Gβ1γ2, constructs Rluc-Gβ1 and Gαs-GFP10 were used to assess the influence of β2AR activation on the Gα/βγ interaction by BRET. In the absence of β2AR activation, a basal BRET signal was detected between Rluc-Gβ1 and Gαs-GFP10 (FIG. 4 c) consistent with the existence of a preassembled G protein heterotrimer. Addition of the β2AR-agonist induced a significant increase in BRET signal, most likely reflecting conformational changes within Gα and Gβγ complexes following receptor-mediated G protein activation.

To assess whether reduction in the receptor-G protein coupling efficacy that follows sustained agonist-stimulation (desensitization) could be detected using the receptor-Gβγ interaction BRET-based assay, the effect of longer term agonist stimulation on the BRET detected between β2AR-Rluc and GFP10-Gγ2 was determined. Although the BRET signal was stable for at least 1 minute, sustained agonist exposure led to a time-dependent reduction in the BRET signal observed that reached 53% of the maximal BRET signal 10 minutes after the initial application of the stimulus (FIG. 4 d, left panel). This progressive decrease in BRET most likely reflects the agonist-promoted desensitization of the receptor, since the loss of BRET signal over time paralleled the reduction in agonist-stimulated cAMP production (FIG. 4 d, right panel). Similarly, the concentration-dependent increase in agonist-promoted BRET was considerably blunted in cells pre-treated for 1 hour with the agonist isoproterenol (FIG. 4 e). The assay also allows the reliable study of the kinetics of desensitization as it occurs in living cells. Indeed, when similar BRET experiments were conducted with the α2AAR, a receptor known to undergo slower desensitization⁵³, the loss of BRET signal between α2AAR-Rluc and GFP10-Gγ2 upon stimulation with the α2AAR agonist, UK14304, occurred with slower kinetics, decreasing by only 24% after 10 minutes (as compared with 53% for the β2AR) (FIG. 4 f). The reduced BRET signal observed between the receptor and βγ during the desensitization process is consistent with the known reduced ability of the desensitized receptors to engage G proteins⁵⁴. Whether this results from a reduced ability of the receptor to recruit G proteins or from a diminished conformational switch within a pre-assembled receptor/G protein complexes remains to be investigated.

The results presented here clearly establish receptor/G protein BRET assays as reliable real-time biosensors for receptor-mediated G protein activation in living cells.

In recent years, several sophisticated in vitro approaches, such as surface plasmon resonance⁵⁵ or flow cytometry⁵⁶, have been developed to measure real-time interactions between GPCRs and their cognate G proteins. Because they rely on the use of purified proteins, these methods permit the accurate determination of the kinetics and affinity of interactions between select members of the ligand-receptor-G protein ternary complex. However, because they are performed outside the natural environment of the cell, such techniques cannot integrate the possible regulatory influence of other cellular factors. Moreover, the immobilization of one of the partners on a cell surface may restrict its movements and influence the dynamics of the interaction. It follows that the real time BRET assay provides the first method allowing the direct kinetic measurement of the receptor/G protein interaction in their natural environment, the living cells.

The BRET-based approach allowed monitoring the direct interaction between GPCR-Rluc or -GFP10 and each of the individual components of the heterotrimeric G proteins (Gαs-GFP10, Gαi-Rluc, Gβ1-GFP10, Gγ2-GFP10). In addition, the selectivity of interaction between a receptor and a given Gα subunit can be indirectly assessed using the receptor/Gβ1 or receptor/Gγ2 sensors by assessing the ability of specific unmodified Gα isoforms to potentiate the agonist-promoted BRET responses. Based on these results, there is reason to believe that the selectivity of Gβ and Gγ isoforms can also be determined using the receptor/Gα sensor. This should prove to be an important aspect of the method, since establishing the selectivity of interaction between receptors and their cognate G protein isoforms has been a difficult task with currently available in vitro assays⁵⁷.

Extension of the invention: use of three resonance energy transfer (RET) technologies (BRET¹+BRET²+FRET) in a unique assay for the detection of receptor-mediated G protein activation within three independent interactions in the receptor-G protein complex.

Advantage: the measurement of three different but related proteins interactions in a unique assay allows the determination of all aspects of a same biological phenomenon, the receptor-mediated G protein activation. This increases the opportunity to detect consequent modulated-BRET signals that reflect specific G protein activation mechanisms.

Proof of Principle:

I) Technological Basis

Resonance energy transfer technologies, BRET (Bioluminescence resonance energy transfer) or FRET (Fluorescence resonance energy transfer), rely on a common principle: the transfer of energy between a donor and an acceptor that share overlap in emission and excitation spectrum. Based on that concept, different generations of BRET/FRET have been conceived:

BRET¹ is resulting from an energy transfer between the bioluminescent energy donor Renillia Luciferase (Rluc), following degradation of its substrate Coelenterazine h (λem=470 nm), and the fluorescent acceptor, Enhanced Yellow Fluorescent protein (EYFP) (λexc=515 nm; λem=528 nm).

BRET² is a modification of BRET¹ based on the use of another substrate of Rluc, with spectral properties different from that of coelenterazine h. Herein, the BRET² results from an energy transfer between the energy donor Renillia Luciferase (Rluc), following degradation of the substrate DeepBlueC (λem=400 nm), and a different fluorescent acceptor (with compatible spectral properties to be excited), the Green Fluorescent protein (GFP²) (λexc=395 nm; λem=510 nm).

Because YFP and GFF² have different excitation and emission wavelengths, it should be easy to detect both resonance energy transfers in a unique assay, using these two energy acceptors (BRET¹+BRET²) and their selective substrates as energy sources.

FRET is based on the transfer of energy between two fluorophores. Classically, FRET studies rely on CFP/EYFP donor/acceptor pairs. However, because GFP² has an emission spectrum overlapping exactly with the excitation wavelength of EYFP, these two fluorophores could be used as a donor and acceptor of energy respectively in a FRET assay.

Because BRET¹, BRET² and GFP²/EYFP-FRET all share common energy donors or acceptors, these three RET-based approaches were used to detect at the same time (i.e., in three independent interactions in the same assay).

II) Experimental Validation of a BRET¹/BRET²/FRET-Assay

To validate this RET-assays combination, the interaction between the obligatory dimeric receptor GABAbR1-GABAbR2 and the heterotrimeric G protein was assessed.

For that purpose, GABAbR1 (GBR1) and R2 (GBR2) receptors were tagged with GFP² and EYFP respectively, at their C-terminus (Cf. FIG. 12). FRET between GFP2 and EYFP was thus measured to detect dimerization of GABAbR1/GABAbR2 subunits.

Gα_(i1) subunit was tagged with Rluc in its helical domain. Thus, we detected interaction of Gα_(i1)-Rluc with the GABAbR1-GFP2 using BRET2 while the interaction of Gα_(i1)-Rluc with GABAbR2-EYFP was followed using BRET¹.

HEK293T cells were co-transfected with a fixed amount of GBR1-GFP², Gα_(i1)-Rluc and Gβ₁γ₂ while increasing the level of GBR2-EYFP. As shown in FIG. 12, when cells were incubated with specific Rluc substrates (coelenterazine h or DeepBlueC for BRET) or excitated with a specific laser (for FRET), for all generation of RET-technologies, the energy transfer increased hyperbolically as a function of the GBR2-EYFP expression level, indicative of specific interactions between GBR1 and GBR2 receptor subunits as reflected by FRET, between Gα_(i1)-Rluc and GBR1-GFP10 as reflected by BRET², and finally between Gα_(i1)-Rluc and GBR2-EYFP as reflecting by BRET¹.

In conclusion, we demonstrate that it is feasible to measure three different protein interactions in a same population of transfected cells, by using proteins tagged with specific energy donors/acceptors amenable for concomitant BRET¹, BRET² and FRET measurements.

III) Application of BRET¹/BRET²/FRET-Assay for Detection of Multiple Interactions in the Receptor/Gα/Gβγ Complex

As it is currently designed, the single BRET-based biosensor, described previously in the present invention, that we have elaborated for receptor-mediated G protein activation, is solely based on the use of BRET² for the detection of individual protein-protein interactions in the receptor/G protein complex (Gα or Gβ or Gγ interaction with the receptor-Gα and Gβ interactions). Thus, measurement of all protein interactions occurring in the receptor/G protein complex necessities independent BRET assays. The BRET¹/BRET²/FRET-assay presented above could then be advantageously be applied to the BRET-based receptor/G protein interaction assay for the detection of all the interactions in the receptor/G protein complex but in a unique assay.

FIG. 13 illustrates the idea behind an experiment where receptor-EYFP, Gα-Rluc and GFP²-Gβ or -Gγ would be co-transfected all together. FRET signal would reflect interaction between receptor and G protein βγ subunits (GFP²-Gβ or -Gγ receptor-EYFP). BRET¹ would allow measurement of receptor/Gα subunit (receptor-EYFP/Gα-Rluc) while interaction between G protein subunits would be followed by BRET² (Gα-Rluc/GFP²-Gβ or -Gγ).

Example 1 Configuration of the BRET Partners Used to Monitor Receptor-Mediated G Protein Activation

To monitor the interactions between GPCRs and their cognate G proteins and among the G proteins subunits, BRET¹ and BRET² signals were measured between the different partners in living cells in the presence and absence of selective receptor ligands. Receptors, Gα_(i1), Gβ₁ and Gγ₂ were fused to BRET energy donor or acceptor (FIG. 15 a). The receptors (α₂AR, β₂AR and CRLR) were fused at their carboxyl tail to the Renilla reniformis luciferase (Rluc) or Aequorea victoria green fluorescent proteins (GFP2, GFP10 or YFP, depending on the partners considered). Gβ₁ and Gγ₂ were fused to their N-terminus to either Rluc or GFP10. For Gα_(i1), two different constructs were engineered so as to allow a better monitoring of the relative movements of the partners. Rluc was inserted in connecting loops located at opposite sides within the helical domain of the protein (FIG. 15 b). One of the insertion site between L91 and K92 in the loop connecting helices A and B (Gα_(i1)-91Rluc) has previously been described⁶, whereas the second, located between E122 and L123 in the loop connecting helices B and C (Gα_(i1)-122Rluc), is presented here for the first time. The same flexible linker (SGGGGS) was used at both N- and C-terminus of the RLuc for the two positions. (Structures are represented in FIG. 16.)

The receptor fusion proteins are functional, as reflected by their ability to bind their selective ligands and to regulate adenylyl cyclase activity (data not shown). As previously reported⁸, the attachment of Rluc or GFPs to the N terminus of both Gβ₁ and Gγ₂ constructs did not affect their targeting to the plasma membrane or their ability to activate the G protein-gated inward rectifying K+ channel (data not shown). Similarly to what was previously found for the insertion of a GFP at position 91⁶, the introduction of Rluc at either position 91 or 122 of Gα_(i1) was also well tolerated. Immunofluorescence confocal microscopy revealed that both Gα_(i1)-91Rluc and Gα_(i1)-122Rluc were properly targeted to the plasma membrane when co-expressed with their complementary Gβ₁γ₂ subunits (FIG. 17). They were also found to be functional, as illustrated by their ability to enhance the α_(2A)AR-promoted inhibition of adenylyl cyclase activity (FIG. 18).

The configurations of the BRET partners used to probe the receptor-mediated G protein activation are illustrated in FIG. 19. The interactions between the receptors and Gβγ subunits were measured in cells co-expressing receptor-Rluc and GFP10-Gβ₁ (FIG. 19 a) or GFP10-Gγ₂ (FIG. 19 b) while receptor-Gα interactions were monitored in cells co-expressing receptor-GFP and Gα_(i1)-91Rluc (FIG. 19 c) or Gα_(i1)-122Rluc (FIG. 19 d). For the interactions among the G protein subunits, BRET was measured between GFP10-Gγ₂ and Gα_(i1)-91Rluc (FIG. 19 e) or Gα_(i1)-122Rluc (FIG. 19 f in the presence of untagged receptors. In all cases, tagged-G protein subunits were co-expressed with their complementary untagged subunits to maintain stoichiometric expression of the three subunits in order to assure their proper targeting to the plasma membrane⁸.

Example 2 Receptor-Ligand Binding Promotes Conformational Rearrangement within Receptor/G Protein Complexes

BRET² was monitored between the human α_(2A)-adrenergic receptor (α_(2A)AR-Rluc or α₂AR-GFP2) and each of the G protein subunits (GFP10-Gβ₁; GFP10-Gγ₂; Gα_(i1)-91Rluc or Gα_(i1)-122Rluc). As shown in FIG. 20, a basal BRET signal was detected in all cases, indicative of a constitutive receptor-Gαβγ complex that may reflect pre-association⁸. The specificity of this interaction, was confirmed by the observation that no significant signal was detected between a form of the transmembrane protein CD8 fused to Rluc or GFP2 at its C-terminus⁸ and any of the GFP2 or Rluc tagged-G protein subunits (data not shown). Stimulation of the receptor with the full agonist, UK14,304, significantly increased the BRET detected for the α_(2A)AR-Rluc/GFP10-Gβ₁, α_(2A)AR-Rluc/GFP10-Gγ₂ and Gα_(i1)-91Rluc/α_(2A)AR-GFP2 pairs (FIG. 20 a). In contrast, the BRET was significantly reduced by the agonist stimulation for the Gα_(i1)-Rluc122/α_(2A)AR-GFP2 pair (FIG. 20 a). In all cases, the changes in BRET signals were blocked by a pre-treatment with pertussis toxin indicating that they reflected receptor-mediated G protein activation (FIG. 21).

The kinetics of agonist-promoted changes were then assessed by monitoring in real time the evolution of the BRET¹ signal between α_(2A)AR-Venus and either Gα_(i1)-91Rluc, Gα_(i1)-122Rluc or Rluc-Gγ₂. As shown in FIG. 22, constitutive BRET signals are stable under basal conditions but addition of the agonist UK14,304, promoted rapid increases in the BRET for the Rluc-Gγ₂/α_(2A)AR-Venus and Gα_(i1)-91Rluc/α_(2A)AR-Venus pairs and a similarly rapid decrease in BRET¹ for the α_(2A)AR-Venus/Gα_(i1)-122Rluc pair. In all cases, the BRET levels reached following agonist stimulation remained constant for at least 12 seconds with no evidence of a return toward basal values, indicating that, at any given time during the early phase of activation, a significant fraction of the receptor is engaged in a ternary (agonist-receptor-G protein) complex. These results are difficult to reconcile with the classical model suggesting that a rapid dissociation of the G protein subunits from the receptors follows their initial agonist-promoted engagement. Indeed, in such a model the changes in BRET signals should be independent of the relative position of the energy donor on the Gα subunit. It follows that the opposite BRET changes detected when using Gα_(i1)-122Rluc or Gα_(i1)-91Rluc as the energy donor most likely reflect an agonist-promoted conformational rearrangement within the receptor-Gαβγ complex that is differentially sensed depending on the position of the energy donor. It may alternatively be suggested that the agonist-modulated BRET signal reflects the equilibrium between the association and dissociation phases of the activation cycle and that the position of the tags could affect the kinetics of the cycle. For instance, the GFP10-Gβ₁, GFP10-Gγ₂ and Gα_(i1)-91Rluc probes could detect the association, whereas Gα_(i1)-122Rluc could detect the dissociation. This is, however, unlikely since both Gα_(i1)-91Rluc and Gα_(i1)-122Rluc led to an increase in BRET signal following the activation by two other GPCRs, the human β₂-adrenergic receptor (β₂AR-GFP10) (FIG. 23 a) and the human calcitonin gene related peptide receptor (CGRP-R=CRLR-YFP+RAMP1) (FIG. 23 b). The directions of the BRET changes are therefore not a reflection of intrinsic kinetic properties of Gα_(i1)-91Rluc and Gα_(i1)-122Rluc but rather appear to reflect specific conformational rearrangements that are differentially sensed by the two positions of the BRET partners.

The observation that the BRET between receptors and Gα_(i1)-122Rluc varied in opposite directions following activation depending on the receptor considered further indicates that they reflect relative movements between the carboxyl tail of the receptor and the helical domain of Gα. The different orientation of the change observed for the αAR when compared to the βAR and the CRLR is not surprising given the much shorter carboxyl tail of the αAR (23 aa compared to 87 and 78 respectively). Consistent with this interpretation, a decrease in BRET was also observed between another receptor with a short tail the short tail, the αAR-GFP2 and Gα-122Rluc (FIG. 24).

The effect of a panel of α_(2A)AR-selective ligands on the receptor/G protein BRET² signals was investigated next. As shown in FIG. 25, qualitatively similar patterns of modulation were observed when the molecules were tested on the α_(2A)AR-Rluc/GFP10-Gβ₁, α_(2A)AR-Rluc/GFP10-Gγ₂ or α_(2A)AR-GFP2/Gα_(i1)-91Rluc. The partial agonists clonidine and dexmedetomidine promoted partial increase in BRET when compared with that induced by the full agonist UK14,304, whereas the antagonists yohimbine and RX821002 had little or no effect. Pretreatment with RX821002 completely blocked the UK14,304-promoted BRET increase, further confirming the pharmacological specificity of the modulations observed. When considering the α_(2A)AR-GFP2/Gα_(i1)-122Rluc pair (FIG. 23), all compounds tested promoted significant decreases in BRET, UK14,304 and dexmedetomidine, leading to the larger responses. The observation that not only the agonists but also the antagonists RX821002 and yohimbine promoted a decrease in the BRET between the receptor and Gα_(i1)-122Rluc further confirms that the observed reduction in BRET does not necessarily reflect a dissociation of the G protein from the receptor but most likely a conformational change within the complex since antagonists would not be predicted to promote G protein dissociation.

Example 3 Ligand-Promoted Conformational Changes within Preformed Receptor/Gαβγ Complexes

The following experiments were designed to determine whether the agonist-promoted structural reorganization of the receptor/G protein complex occurred as a result of an active recruitment of the G proteins to the receptor. Since an active recruitment presupposes that agonist stimulation increases the affinity of the receptor for the G protein subunits, BRET titration assays between α_(2A)AR-Rluc and GFP10-Gβ₁ or GFP10-Gγ₂ and between Gα_(i1)-91Rluc or Gα_(i1)-122Rluc and α_(2A)AR-GFP2 were carried out in the presence and absence of UK14,304. BRET₅₀ (the GFP/Rluc ratio leading to 50% of the maximal BRET signal) derived from such titration curves is used as relative indicator of the affinity between partners^(23, 24).

As shown in FIG. 26 a, agonist treatment led to an increase (α_(2A)AR-Rluc/GFP10-Gβ₁, /GFP10-Gγ₂ or Gα_(i1)-91Rluc/α_(2A)AR-GFP2) or a decrease (Gα_(i1)-122Rluc/α_(2A)AR-GFP2) of the maximal BRET but did not affect the BRET₅₀, indicating that receptor activation did not influence the relative affinity of the receptor for Gα, Gβ or Gγ (see table in FIG. 26 a) This is not unique to the α_(2A)AR since similar results were obtained when assessing the relative affinity between β₂AR-Rluc and GFP10-Gβ₁ or GFP10-Gγ₂ (FIG. 26 b). The lack of change in the BRET₅₀ observed between the receptor and the G protein subunits contrasts with the significant agonist-promoted decrease in BRET₅₀ observed when assessing the recruitment of YFP-βarrestin-1-383T²⁵ to the β₂AR-Rluc (FIG. 26 c), thus confirming that an active recruitment can be detected by a leftward shift of the BRET titration curve. The lack of such a shift in the BRET titration curves between the receptors and Gα, Gβ or Gγ subunits suggests that a significant fraction of the receptor and Gαβγ exists as a preformed complex which is subject to structural rearrangements upon ligand binding. The existence of a pre-assembled receptor/G protein complex that undergoes conformational rearrangements upon receptor activation is supported by the observation that Gβ₁ and Gγ₂ can be co-immunoprecipitated with β₂AR in the absence of agonist stimulation and that the extent of association was not affected by receptor activation (FIG. 26 d). Altogether, this data does not support active agonist-dependent recruitment of Gαβγ to the receptor and may be more consistent with a model whereby agonist binding induces conformational changes within a pre-existing receptor/Gβγ complex.

Example 4 Receptor-Promoted Conformational Rearrangement in Heterotrimeric Gαβγ Protein Complex

The effect of receptor activation was then assessed on the Gα/Gβγ interaction in cells co-expressing GFP10-Gγ₂ and either Gα_(i1)-91Rluc or Gα_(i1)-122Rluc in the presence of untagged α_(2A)AR (FIG. 27). The full α_(2A)AR agonist, UK14,304, promoted a reduction of BRET detected between both Gα_(i1)-91Rluc or Gα_(i1)-122 Rluc and GFP10-Gγ₂ (FIG. 27 a). These BRET changes were abolished by a treatment with pertussis toxin confirming that they reflected G protein activation (FIG. 21). The similar modulations obtained with the two Gα_(i1)-Rluc constructs in the Gα_(i1)/Gγ₂ interaction contrast with the effect that the distinct position of the Rluc-tag had on the BRET signals detected when considering the α_(2A)AR/Gα_(i1) interaction (see previous section). To exclude that the dependency on the Gα_(i1) tag position observed for the Gα_(i1)/α_(2A)AR (FIG. 20) but not the Gα_(i1)/Gγ₂ (FIG. 27 a) interaction could result from different experimental conditions, advantage was taken of the possibility of detecting BRET¹ and BRET² concomitantly in the same cells²⁶. For this purpose, Gα_(i1)-91Rluc, α_(2A)AR-Venus and GFP10-Gγ₂ were co-expressed and the two BRET signals were detected following the addition of the luciferase substrates, coelenterazine H or Deep-BlueC, for BRET¹ and BRET², respectively. As shown in FIG. 27 b, activation of the receptor with UK14,304 led to an increase in the BRET between Gα_(i1)-91Rluc and α_(2A)AR-Venus but a decrease in BRET² between Gα_(i1)-91Rluc and GFF₁₀-Gγ₂, confirming the results obtained when the two interactions were monitored independently.

The effect of the panel of α₂AR ligands on the Gα/Gβγ interaction was then examined. As shown in FIG. 27 c, whether the Rluc was inserted in position 91 or 122 of Gα_(i1), the full and partial agonists UK14,304, clonidine and dexmedetomidine decreased the BRET between Gα_(i1)-Rluc and GFP10-Gγ₂. However, as was the case for the α_(2A)AR/Gα_(i1) interaction, the antagonist yohimbine and RX821002 had no effect on the BRET signal between Gα_(i1)-91Rluc and GFP10-Gγ₂ (FIG. 27 c) but promoted a significant decrease in the BRET signal between Gα_(i1)-122Rluc and GFP10-Gγ₂ (FIG. 26 c). Since an antagonist should not lead to G protein subunit dissociation, the decrease in BRET cannot reflect a dissociation of the two subunits but most-likely results from a conformational rearrangement within the G protein heterotrimer.

Example 5 Insight into Structural Rearrangements of Gαβγ Complex

To further explore the structural rearrangements within the Gα_(i1)β₁γ₂ complex upon receptor activation, BRET between Gα_(i1)-91Rluc or Gα_(i1)-122Rluc and GFP10-Gγ₂ was monitored in response to the activation of a panel of receptors (α_(2A)-, β₁- and β₂-adrenergic, dopamine-D1, δ-opioid, muscarinic-M2, chemokine-CXCR4 and -CCR5, calcitonin gene related peptide [CRLR+RAMP1], prostaglandine-EP4, vasoactive intestinal peptide and secretine). As shown in FIG. 28 a, activation of each receptor with their cognate agonists led to BRET reductions for both Gα_(i1)-91Rluc and Gα_(i1)-122Rluc. Although the orientation of the change was the same, whatever the position of Rluc in the Gα_(i1) subunit, the decrease in BRET was more dramatic for the Gαi-91 position, indicating that the relative movements of the AB- and BC-loops away from the amino-terminal of the Gγ₂ are of different amplitude.

In an effort to better understand the nature of the conformational rearrangements between Gγ₂ and Gα_(i1) following activation, a third construct (Gα_(i1)-60Rluc) was engineered where RLuc was introduced in the linker 1 region connecting the helical and GTPase domains of Gα_(i1) (FIG. 15 b). In contrast to what was observed for the 91 and 122 position, all receptors tested promoted an increase in BRET between Gα_(i1)-60Rluc and GFP10-Gγ₂ (FIG. 28 b). This opposite change indicates that while the AB- and BC-loops move away, the linker-1 region gets closer from the amino-terminal of Gγ₂ during the activation process, further supporting that the BRET changes observed do not reflect a complete dissociation of the Gα_(i1)β₁γ₂ complex but rather a reorganization within the complex. The fact that all receptors tested promoted the same pattern of BRET changes between GFP10-Gγ₂ and the three Gα_(i1)-Rluc biosensors suggests that they reflect a common Gi activation process.

DISCUSSION Activation of Pre-Associated α_(2A)AR/Gα_(i1)β₁γ₂ Complex

The detection of a specific basal BRET signal between the α_(2A)AR and either Gα_(i1), Gβ₁ or Gγ₂ clearly indicates that at least a fraction of the receptor exists in pre-associated complexes with Gαβγ in the absence of receptor activation. This is consistent with a significant body of evidence indicating that receptor promoted activation of G proteins does not result only from random collision between proteins but involves more highly organized modules that include pre-coupled receptor/G protein complexes^(27, 28). Therefore, a question raised by the present data is whether the detected changes in receptor/G protein interactions result from the recruitment of G proteins to the activated receptor or from rearrangements within pre-existing receptor/G protein complexes.

Two lines of evidence support the second hypothesis. First, BRET₅₀ values obtained from BRET titration curves did not reveal any change in the apparent affinity of the α_(2A)AR or β₂AR receptor for the G protein subunits following agonist stimulation. Second, depending on the position of the energy donor (Rluc) within Gα_(i1), agonist stimulation of α_(2A)AR-GFP2 led to either an increase (for Gα_(i1)-91Rluc) or a decrease (for Gα_(i1)-122Rluc) in the BRET between receptor and Gα following similar kinetics. An active recruitment of the G protein to the receptor would be predicted to increase the relative affinity of the receptor for the G protein subunits that should be detected by an increase in BRET, independently of the position of the energy donor within Gα_(i1). Thus, the data does not support the occurrence of a dynamic recruitment of the G protein to the receptor during the early event of G protein activation.

The agonist promoted conformational rearrangements within a pre-assembled receptor/G protein complex suggested by the data is also supported by similar results obtained with the Protease Activated Receptor and both Gα₁₂ and Gα_(i1) presented by Ayoub et al (provide citation). These conclusions are consistent with recent computational modeling of the rhodopsin/transducin (Gt) complex. Indeed, analysis of the electrostatic and shape complementarity between the crystal structure of inactive rhodopsin and Gt suggested that light leads to conformational changes within a rhodopsin/Gt supramolecular complex formed prior to the activation of the photoreceptor²⁹.

Consistent with the latter conclusion is the observation that both the agonist-promoted increase (for Gα_(i1)-91Rluc) and decrease (for Gα_(i1)-122Rluc) in BRET between the receptor and Gα_(i1) were stable for extended period of time with no indication of a change in the association status. It could, however, be argued that the stable BRET signals observed following agonist stimulation reflects a new steady-state in the association/dissociation equilibrium and that the different BRET levels reached for Gα_(i1)-91Rluc and Gα_(i1)-122Rluc result from the different position of the energy donor within the associated complex. If it were the case, one would predict that preventing the re-association branch of the cycle would affect the steady-state BRET signals observed following receptor activation and allow the detection of dissociation. Contrary to this prediction, a stable agonist-promoted increase in BRET was also detected between β₂AR-GFP10 and a mutant form of Gα_(i1) (QL-Gα_(i1)-122Rluc) that is unable to hydrolyse GTP and thus should not re-associate following receptor activation (FIG. 29). These results therefore demonstrate that the agonist-promoted changes in BRET reflect conformational changes within metastable receptor/G protein complexes, with no evidence for dissociation. Further supporting the notion that the decrease in BRET signals do not reflect a simple dissociation between the receptor and the G protein subunits is the observation that antagonist that should not promote dissociation also lead to a reduction of the BRET between α_(2A)AR-GFP2 and Gα_(i1)-122Rluc.

The conformational nature of the ligand-modulated BRET signals also applies when considering the interactions between the G protein subunits. Indeed, as was the case for the receptor/G protein interactions, both agonists and antagonists promoted decreases in BRET between Gα_(i1)-91Rluc or Gα_(i1)-122Rluc and GFP10-Gγ₂. Moreover, receptor-promoted activation of the G protein could be detected either as a decrease (for Gα_(i1)-91Rluc or Gα_(i1)-122Rluc) or an increase (for Gα_(i1)-60Rluc) in BRET between Gα_(i1)-Rluc and GFP10-Gγ₂, thus confirming that distinct position of the energy donor can probe the conformational change differentially. The lack of receptor-promoted G protein subunit dissociation that is suggested by the position-dependent changes in agonist-modulated BRET signals is consistent with a previous report by Bünneman et al⁶. In their study, opposite agonist-promoted FRET signals between Gα_(i1)-91YFP and either Gγ₂-CFP (agonist-promoted decrease) or CFP-Gγ₂ (agonist-promoted increase) led them to conclude to the absence of G protein subunit dissociation. In a more recent study, however, the observation that receptor activation promoted a decrease in FRET between Gα_(o)-91YFP and CFP-Gγ₂ led them to conclude that the lack of dissociation may be unique to Gα_(i1) ⁷. Although, the authors considered the possible contribution of distinct conformational rearrangements, they did not formally test this hypothesis by using different fluorophore positions in either Gα₀ or Gγ but yet concluded to a dissociation of the two subunits. This is particularly surprising when considering that the AB-loop primary sequence diverges considerably between Gα_(i1) and Gα₀ (less then 40% identity). Contrasting with this proposed G protein-specific activation process (Gα_(i1) not dissociating from Gβγ whereas Gα₀ does), BRET between Rluc-Gγ and Gα-71 GFP10 increased following stimulation of a panel of receptors (β- and β-adrenergic, dopamine-D1, vasopressin-V2, prostaglandine-EP4, vasointestinal peptide and secretine) (FIG. 30), suggesting that the lack of G protein subunit dissociation in the early step of activation is not unique to Gα_(i1).

This data challenges the classical collision-based model whereby functional interactions between the various partners were believed to reflect true ligand-regulated association/dissociation cycles. However, this model was deduced largely from in vitro assays using purified proteins⁴ in which the cellular factors influencing the dynamics of protein interactions could be easily recreated. More recently, agonist-promoted decreases in FRET between Gα and Gβ or Gγ in Dictyostelium discoideum and Saccharomyces cerevisiae, respectively, were interpreted as evidence of receptor-promoted dissociation of the G protein complex in living cells^(30, 31). Although a loss of FRET is consistent with dissociation, it can also reflect conformational rearrangements that promote an increase in the distance between the fluorophores inserted in the Gα and Gβ or Gγ subunits. Based on primary sequence comparison, the position used to insert the GFP variants into the Dictyostelium discoideum Gαsubunit corresponds to the position 91 used to insert Rluc into the mammalian Gα_(i1) in the present study. Consistent with the decrease in FRET previously observed, receptor activation led to a decrease in BRET between Gα_(i1)-91Rluc and GFP10-Gγ₂. Only the monitoring of a distinct position within Gα_(i1) (Gα_(i1)-60Rluc) permitted the observation of the increase which suggested that a conformational change rather than a dissociation was being detected. Consistent with the notion that subunit dissociation may not be occurring in the early step of G protein activation, is the observation by Klein et al⁵ that expression of a construct covalently attaching Gα to Gβ rescued α-mating factor responsiveness in a Gβ deficient yeast strain, demonstrating that physical dissociation of the subunits is not a pre-requisite for G protein activity.

The data presented above does not permit the exclusion of the occurrence of receptor-promoted dissociation of the complex but clearly demonstrates that structural rearrangements within a stable α_(2A)AR/Gα_(i1)β₁γ₂ complex can be monitored in the early steps of the activation process.

Structural Rearrangements of the Receptor/G Protein Complex Following Receptor Activation

The efficacy of resonance energy transfer depends on both the distance and the orientation between energy donors and acceptors inserted in specific positions within macromolecular complexes. Thus, in combination with ether types of structural information such as crystal coordinates and molecular modelling, variations in FRET or BRET efficacy have been used to monitor both intra- and inter-molecular conformational rearrangements and to propose or refine dynamic rearrangements within structural models^(18, 20-22). Applying this reasoning to the changes in BRET signals observed between the different positions within the receptor/G protein complexes allows inferences to be made concerning the conformational changes occurring during the initial steps of activation. When considering the relative movement of the receptor and G protein in relation to one another, the data suggests that the receptors' carboxyl tails, the AB-loop of the Gα_(i1) helical domain and the N-terminal of Gγ₂ get closer following receptor activation. Indeed, for the three receptors tested (α_(2A)AR, β₂AR and CGRP receptor), agonist stimulation promoted an increases in the BRET signal between the receptor's carboxyl tails and both Gα_(i1)-91Rluc and GFP10-Gγ₂. Based on the crystal structure of the inactive Gα_(i1)β₁γ₂ heterotrimer³², the N-terminal of Gγ₂ is in close apposition to the AB-loop of the Gα_(i1) helical domain (FIG. 31 a). It follows that receptor activation most-likely leads to a rearrangement that brings the bottom part of the heterotrimer closer to the receptors' carboxyl tail. In contrast with this conformational rearrangement that was perceived independently of the length of the receptor's carboxyl tail, the relative movement of the BC-loop of the Gα helical domain was sensed differentially depending on the receptor considered: a decrease in BRET was detected between Gα_(i1)-122Rluc and the α_(2A)AR and α_(2B)AR short C-tail whereas an increase was observed with the β₂ AR and CGRP longer C-tails.

Given the lack of structural information about the active receptor conformation and the receptor/G protein interface, it is difficult to propose a detailed conformational reorganization clearly describing the relative movement of the G protein vs the receptor's carboxyl-tail during activation. However, the observation that PTX pre-treatment greatly blunted agonist-promoted BRET changes between the receptor and all the tested positions within the G protein heterotrimer clearly demonstrates that the BRET changes reflects, at least in part, the relative movement of the G protein subunits during activation. The comparison of the BRET changes detected by the different positions within the G protein heterotrimer also allows inferences to be made about the conformational changes stabilized by various ligands. For instance, whereas the partial agonist dexmedetomidine promoted a partial increase in BRET between α_(2A)AR and either Gβ₁, Gγ₂ or the position 91 of Gα_(i1) (when compared with the increase promoted by the full agonist UK14,304), the decrease in BRET detected between the receptor and the position 122 of Gα_(i1) were of similar amplitude for dexmedetomidine and UK14,340. This clearly indicates that the partial agonist promotes conformational changes that are distinct from those stabilized by the full agonist. These results therefore do not support the notion that partial agonism results from the stabilization of only a fraction of the receptor into an active conformation identical to that promoted by the full agonist. Therefore, the use of multiple BRET sensor sites within the receptor-G protein complex permits the detection of ligand-specific conformational changes that most likely reflect distinct “textures” in ligand efficacy^(33, 34).

Structural Rearrangements of the Heterotrimeric G Protein Complex Following Receptor Activation

When considering the movement of Gγ₂ relative to the three different insertion sites probed within Gα_(i1) (60, 91 and 122), the profiles of agonist-promoted BRET changes were identical for the twelve receptors tested. In all cases, receptor activation led to an increase in BRET between Gγ₂ and Gα_(i1)-60 and a decrease between Gγ₂ and either Gα_(i1)-91 or Gα_(i1)-122. Such conservation of the BRET change profiles among a large selection of GPCRs strongly indicates that the changes observed reflects a common structural rearrangements characteristic of G protein activation.

Based on the crystal structure of Gα_(i1)β₁γ₂, the positions of the energy donors and acceptors allows the monitoring of the relative movements between the Gγ₂N and either, AB- (position 91) or BC- (position 122) loops within the helical domain or the linker 1 (position 60) connecting the helical domain to the GTPase domain of Gα_(i1). Taken as a whole, the agonist-promoted BRET changes between the different sites suggest that the helical domain rotates away while the linker 1 region gets closer to Gγ₂N (FIG. 31 b). This is consistent with the flexible linker 1 region acting as a hinge during the opening of the helical domain. Such a conformational changes that stabilizes GαH in an open conformation is required to create a in exit route for the nucleotide from the GTPase domain following activation. This finding is not inconsistent with the “lever arm” model of G protein activation^(14, 15), since the movement of the GαN could lead to the opening of Gβγ away from Gα. However, the data indicates that the opening of the GαH during activation is not accompanied by a complete dissociation of Gα from Gβγ, since the Gα linker 1 region appears more closely associated to the Gγ following receptor stimulation. In that sense, the data is entirely compatible with the more recent “gear shift” model proposed by Cherfils et al. which is based on the homology with the activation process of small G proteins by their guanine nucleotide exchange factors¹². In that model, the β-propeller region of Gβ is proposed to tighten its interaction with the Gα GTPase domain in the nucleotide empty state promoted by receptor activation. Such closer packing between the Gα GTPase domain and the Gβγ dimer is entirely consistent with the increased BRET observed between GγN and the linker 1 region of Gα_(i1) (FIG. 28 b). Also according to the model, GγN engages a gear that displaces GαH as a rigid body away from the GTPase domain, a movement that should also result in a greater separation between GαH and Gγ. This prediction is in agreement with the decrease in BRET observed between Gγ₂ and both the AB- and BC-loops of Gα_(i1)H (FIG. 28 a). The data is also entirely consistent with a model based on molecular dynamic simulation of transducin, suggesting that nucleotide exchange mechanism of Gt is similar to that of small G proteins¹³.

Taken together the results are consistent with a model whereby receptor activation promotes a “gear-shift”-like reorganization of a preassembled receptor/G protein complex that leads to the opening but not the dissociation of the Gα/βγ interface. Such opening allows GαH to move away form the Gα-GTPase domain, thus facilitating nucleotide exit. It follows that the BRET changes detected between Gγ₂ and the various positions in Gα_(i1) may correspond to the transition toward the nucleotide empty state. It may not be surprising that the dynamic changes monitored by BRET reflect the formation of this transient state since the crystal structure of the GTP- and GDP-bound state revealed only local changes in the nucleotide binding pocket that should not lead to BRET changes when considering the position of the energy donor and acceptor used in the present study. Although consistent with the “gear-shift” model the data may also be compatible with other structural reorganizations of the heterotrimer involving a relative rotation or clamp-like opening of the entire Gα subunit away from Gβγ such that the linker 1 region gets closer while the helical domain gets further from the N-terminus of Gγ.

Although the present invention has been described by way of specific embodiments and examples thereof, it will be apparent to persons of skill in the art that modifications may be made to it without departing from its spirit, scope and nature. For example, while the embodiments have been exemplified with G protein coupled receptors, the invention should also be extended to the tyrosine kinase and cytokine receptor families given the recent findings that these receptors can transduce cell signaling through heterotrimeric G proteins as well.

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What is claimed is:
 1. A bioluminescence resonance energy transfer (BRET) technology-based bimolecular biosensor, comprising: 1) a G protein coupled receptor (GPCR) fused to a bioluminescent molecule or a fluorescent molecule; and 2) a G protein subunit that is part of a G protein heterotrimeric complex fused to a bioluminescent molecule or a fluorescent molecule, said subunit being selected from the group consisting of Gα, Gβ and Gγ, wherein when the GPCR is fused to a bioluminescent molecule, the G protein subunit is fused to a fluorescent molecule, and when the GPCR is fused to a fluorescent molecule, the G protein subunit is fused to a bioluminescent molecule; and wherein energy transferred by the bioluminescent molecule and accepted by the fluorescent molecule can be monitored.
 2. The BRET technology-based bimolecular biosensor as defined in claim 1, wherein said bioluminescent molecule is luciferase and said energy acceptor is Green Fluorescent Protein (GFP) or a GFP variant.
 3. The BRET technology-based bimolecular biosensor as defined in claim 2, wherein said bioluminescent molecule is Renilla luciferase (Rluc) and said GFP variant is GFP10, GFP2, eYFP or eYFPvenus.
 4. The BRET technology-based bimolecular biosensor as defined in claim 1, wherein said G protein subunit is Gα.
 5. The BRET technology-based bimolecular biosensor as defined in claim 4, wherein said Gα subunit is tagged at position 91 with luciferase and is encoded by an expression vector having the nucleotide sequence of SEQ ID NO:
 2. 6. The BRET technology-based bimolecular biosensor as defined in claim 4, wherein said Gα is Gα_(s), Gα₀, Gα_(i1), Gα_(i2), Gα_(i3), Gα_(q), Gα₁₁ or Gα₁₃.
 7. The BRET technology-based bimolecular biosensor as defined in claim 4, wherein said Gα subunit is tagged at position 60 with luciferase and is encoded by an expression vector having the nucleotide sequence of SEQ ID NO:
 1. 8. The BRET technology-based bimolecular biosensor as defined in claim 4, wherein said Gα subunit is tagged at position 122 with luciferase and is encoded by an expression vector having the nucleotide sequence of SEQ ID NO:
 3. 9. The BRET technology-based bimolecular biosensor as defined in claim 4, wherein said Gα subunit is tagged at position 122 with luciferase and is encoded by an expression vector having the nucleotide sequence of SEQ ID NO:
 3. 10. The BRET technology-based bimolecular biosensor as defined in claim 4, wherein said Gα subunit is tagged at position 91 with luciferase and is encoded by an expression vector having a degenerate nucleotide sequence of the nucleotide sequence of SEQ ID NO:2, and said degenerate nucleotide sequence encoding the same Gα subunit as SEQ ID NO:2.
 11. The BRET technology-based bimolecular biosensor as defined in claim 4, wherein said Gα subunit is tagged at position 60 with luciferase and is encoded by an expression vector having a degenerate nucleotide sequence of the nucleotide sequence of SEQ ID NO:1, and said degenerate nucleotide sequence encoding the same Gα subunit as SEQ ID NO:1.
 12. The BRET technology-based bimolecular biosensor as defined in claim 4, wherein said Gα subunit is tagged at position 122 with luciferase and is encoded by an expression vector having a degenerate nucleotide sequence of the nucleotide sequence of SEQ ID NO:3, and said degenerate nucleotide sequence encoding the same Gα subunit as SEQ ID NO:3.
 13. The BRET technology-based bimolecular biosensor as defined in claim 1, wherein said G protein subunit is Gβ.
 14. The BRET technology-based bimolecular biosensor as defined in claim 13, wherein said Gβ is Gβ1.
 15. The BRET technology-based bimolecular biosensor as defined in claim 1, wherein said G protein subunit is Gγ.
 16. The BRET technology-based bimolecular biosensor as defined in claim 15, wherein said G protein subunit is Gγ2.
 17. The BRET technology-based bimolecular biosensor as defined in claim 1, wherein: 1) said GPCR is a mammalian GPCR; and 2) said G protein subunit is a mammalian G protein subunit.
 18. A bioluminescence resonance energy transfer (BRET) technology-based bimolecular biosensor, comprising: 1) a first subunit consisting of a Gα protein subunit fused to a bioluminescent molecule or a fluorescent molecule; and 2) a second subunit consisting of a Gβ or Gγ protein subunit fused to a bioluminescent molecule or a fluorescent molecule, wherein when the first subunit is fused with a bioluminescent molecule, said second subunit is fused with a fluorescent molecule, and when the first subunit is fused with a fluorescent molecule, said second subunit is fused with a bioluminescent molecule, and wherein energy is transferred by the bioluminescent molecule and accepted by the fluorescent molecule when the first subunit is in the presence of the second subunit and said energy transfer can be monitored.
 19. The BRET technology-based bimolecular biosensor as defined in claim 18, wherein said Gα subunit is tagged at position 60 with luciferase and is encoded by an expression vector having the nucleotide sequence of SEQ ID NO:
 1. 20. The BRET technology-based bimolecular biosensor as defined in claim 18, wherein said Gα subunit is tagged at position 91 with luciferase and is encoded by an expression vector having the nucleotide sequence of SEQ ID NO:
 2. 21. The BRET technology-based bimolecular biosensor as defined in claim 18, wherein said Gα protein subunit is a Gα_(s), Gα₀, Gα_(i1), Gα_(i2), Gα_(i3), Gα_(q), Gα₁₁ or Gα₁₃ protein subunit.
 22. The BRET technology-based bimolecular biosensor as defined in claim 18, wherein said Gβ protein subunit is a Gβ1 protein subunit.
 23. The BRET technology-based bimolecular biosensor as defined in claim 18, wherein said Gγ protein subunit is a Gγ2 protein subunit.
 24. The BRET technology-based bimolecular biosensor as defined in claim 18, wherein said Gα is tagged with luciferase and said Gβ or Gγ protein subunit is tagged with a fluorescent molecule.
 25. The BRET technology-based bimolecular biosensor as defined in claim 18, wherein said Gα is tagged with a fluorescent molecule and said Gβ or Gγ protein subunit is tagged with a luciferase.
 26. The BRET technology-based bimolecular biosensor as defined in claim 18, wherein said G protein subunits are mammalian G-protein subunits.
 27. The BRET technology-based bimolecular biosensor as defined in claim 18, wherein said Gα subunit is tagged at position 60 with luciferase and is encoded by an expression vector having a degenerate nucleotide sequence of the nucleotide sequence of SEQ ID NO:1, and said degenerate nucleotide sequence encoding the same Gα subunit as SEQ ID NO:1.
 28. The BRET technology-based bimolecular biosensor as defined in claim 18, wherein said Gα subunit is tagged at position 91 with luciferase and is encoded by an expression vector having a degenerate nucleotide sequence of the nucleotide sequence of SEQ ID NO:2, and said degenerate nucleotide sequence encoding the same Gα subunit as SEQ ID NO:2.
 29. The BRET technology-based bimolecular biosensor as defined in claim 18, wherein said Gα subunit is tagged at position 122 with luciferase and is encoded by an expression vector having a degenerate nucleotide sequence of the nucleotide sequence of SEQ ID NO:3, and said degenerate nucleotide sequence encoding the same Gα subunit as SEQ ID NO:3. 